It is now almost 30 years since Sadamichi Hirasawa entered death row. His eyesight is failing, friends say, and he can hardly see the paints and easel he is allowed to keep, tools with which he made a living before his arrest in 1948. He has withered to 80 pounds and needs the support of other people to walk.

Hirasawa is 93. In his twin cells at a prison in Sendai city, he is believed to hold the world record for time on death row. Since the Japanese Supreme Court affirmed his sentence in 1955, 33 successive justice ministers have declined to put their signatures and seal on his execution order, and he has continued to live.

Hirasawa's friends on the outside take that as proof of official doubt of his guilt. But in the same period, the Japanese criminal justice system has rejected 17 petitions for a new trial and four for a pardon. The bureaucracy's plan, friends say, is to keep him locked up until he dies a natural death. So far, Hirasawa has not cooperated.

The battle to keep the white-bearded man in the public eye is led by the Save Hirasawa Committee. For decades, it has been arguing that Japanese police made a false arrest in the bizarre poisoning death of 12 persons in a Tokyo bank 37 years ago.

In their view, U.S. military officers administering the 1945-51 occupation of Japan probably prevented arrest of the real culprit, most likely a veteran of a secret germ warfare unit of the disbanded Imperial Army, due to political concerns.

Now the committee is trying again. With the 30th anniversary of the Supreme Court ruling to come up on May 7, it has petitioned to invoke a clause in the penal code that says any death sentence not carried out within 30 years is void. It has never been invoked before.

"We think this spring is our last chance," says Takehiko Hirasawa, a 26-year-old man who was formally adopted by the prisoner two years ago, took his name and now leads the fight. "If we miss it, he will almost certainly die in prison."

The name change served both to underline personal commitment and to let the fight go on, if necessary, after the prisoner dies. Only a member of the prisoner's immediate family can sign papers to do so.

The younger Hirasawa grew up with the case. He is continuing a battle begun by his real father, the late journalist Tetsuro Morikawa, who got interested in the case in the 1940s and stuck with it, once going to prison on perjury charges growing out of a hearing.

The adopted son is today one of the few people who sees Hirasawa. The Justice Ministry routinely rejects journalists' requests. He says the prisoner is frail, but still lucid. "He is talking more and more about getting out in the spring," he says. "He wants to go the graves of his parents."

The ministry refuses to comment on the case. But under questioning last year in the Diet, or national legislature, officials said the 30-year provision was intended for people who escape from prison before execution, not those who stay put to await it.

The Japan Socialist Party, the main opposition force in the Diet, is trying to keep the request alive. "After 30 years," says party human rights spokesman Kazuo Okada, "the statute of limitations should be applied. If that is not possible, he should be given clemency. We want to help him."

Hirasawa's case crops up in the newspapers here from time to time, a reminder that Japan has capital punishment, by hanging, and that many Japanese, apparently including some in the government, feel uneasy with it.

At the end of 1983, there were 28 persons awaiting execution in Japan. That is the latest figure available, because the Justice Ministry is secretive about this subject and discloses the death row population only once a year, in its annual report. Executions, believed to occur about once a year, take place without notice or confirmation, sometimes, according to Amnesty International, not even for family members.

By American standards, Japan's legal system is stacked in favor of the prosecution. The police are widely perceived to always get their man. In 1982, for instance, they managed a 97 percent arrest rate on homicides. Almost everyone brought to trial is convicted.

But in the past two years, faith in the system has been shaken by three widely publicized cases in which prisoners were cleared of crimes for which they had earlier been sentenced to die. It has strengthened a campaign against capital punishment, although it is still nowhere near success.

Hirasawa's supporters maintain that for 30 years, government officials have had doubts about his guilt. But with each additional year he spends in prison, officials feel less able to own up to the system's mistake, they say.

Hirasawa was convicted of one of the most sensational murders of postwar Japan, the 12 deaths in the "Imperial Bank Incident" of Jan. 26, 1948. Shortly after 3 p.m. that day, a stout, middle-aged man wearing a government health inspector's armband and carrying a medical case knocked on the door of the now-defunct bank's branch in downtown Tokyo.

The branch had just closed. But employes admitted him after he presented a calling card identifying himself as Dr. Shigeru Matsui and explained that a man infected with dysentery had visited the bank earlier in the day. The authorities, fearful of an epidemic, wanted everyone in the bank to take a dose of preventive medicine immediately, he said.

Sixteen persons assembled, including the bank messenger, his wife and two children, who lived on the premises. Teacups were brought out. The "doctor" pulled a bottle from his bag and poured liquid into each one. At his direction, the 16 raised cups to their lips and bolted the liquid in unison.

Within moments, they were writhing on the floor, victims of potassium cyanide. The murderer quickly gathered 160,000 yen from an open safe and disappeared.

Ten persons were dead by the time doctors arrived. Another two died at the hospital. The dead included the messenger and his entire family. But four persons survived and gave the police a description that started one of the most intensive manhunts in recent Japanese history.

Seven months later, police arrested Hirasawa, at that time 56. A professional painter with an aristocratic bearing, he lived on Hokkaido Island and had built a reputation in the artistic world as a specialist in tempera.

Hirasawa had once received a name card from the real Dr. Matsui. A check that was stolen from the bank during the crime had been endorsed with handwriting said to be identical to Hirasawa's. He had made a large bank deposit shortly after the crime but could not explain the source of the money.

Several days after his arrest, he attempted suicide. Following that failure, he confessed in detail. But on being arraigned, he recanted, saying the police had wrung the statement from him during marathon interrogation.

He stuck to his claim of innocence through the trial but was convicted and sentenced to death. In 1955, he exhausted his appeals when the Japanese Supreme Court upheld his conviction and sentence, and the clock began ticking on death row.

Several times since then, his spirits have been raised by murmurings from the Justice Ministry of release. In 1974, he was moved to a hospital amid speculation that special clemency would come. It did not. In 1980, talk reached the point that the support committee rented a house for him in Tokyo. The lease ran out.

Today, as a condemned man, he holds some special privileges at his prison 180 miles north of Tokyo. His adopted son says one of his cells is for sleeping -- he has a hospital-type bed -- and one for painting. He dresses in a dark blue kimono. He has completed 1,320 works of art in prison.

The Save Hirasawa Committee contends that no material evidence was ever presented to link him to the crime and that two survivors who identified him as the murderer at the trial initially would say only that he resembled the man.

More telling, they say, is that it was never properly explained how he would have obtained the poisons used, or been so familiar with dosage, speed of effects, and the psychology of making 12 persons unwitting assistants in their own murders.

In the months following the killings, they say, police focused on theories that the murderer was a former member of "Unit 731" of the Imperial Army. This infamous group conducted experiments with germ agents and other poisons in China during World War II, sometimes using live prisoners of war.

Most Japanese were unaware of it existence until the mid-1970s, when newspapers disclosed its history and U.S. occupation authorities' decision not to prosecute its members as war criminals. Instead, they were interrogated in secret on their research and allowed to return to normal lives.

The technique used in the bank murders suggests that the man had killed with poison previously, they say. "The murderer was clearly aware that this poison would be effective in one minute," says the younger Hirasawa. "He was expert at killing people in large numbers."

Yet police never arrested anyone from the unit -- because of orders from above, he contends. In addition, he says, the Yomiuri Shimbun newspaper began investigating this theory but was told to stop. Such orders certainly would have originated with U.S. authorities, he contends, to keep the unit secret.