A Japanese version of the Mafia is moving in on Hawaii and the western U.S. mainland, bringing amphetamines, prostitution and new gambling rings, Attorney General William French Smith said yesterday.

Smith said there is "great concern" within the administration "that this not become another network for feeding drugs into this country."

Smith returned Nov. 10 from a three-week trip discussing drug-control issues with officials in six nations, including Japan, where he said the subject was a proposed mutual assistance treaty.

"We have evidence that Japanese organized crime is becoming active in Hawaii," he said, "buying into legitimate businesses, coming in with tourist groups." He said the criminal group, called the Yakuza, is active "not only in Hawaii but also on the West Coast and even beyond."

The proposed treaty would put a systematic exchange of intelligence information in operation "before the Mafia gets a foothold," Smith said.

He added that the 1,000 new FBI agents he is assigning to drug work will only bring the task force back up to 1976 levels. He is reassigning them away from "lower priority" work such as solving bank robberies, he said, and has asked for an additional $130 million in his 1984 budget proposal to pay for it.

Smith discounted the possibility that political and police corruption is so widespread as to cripple any international attack on drugs. "I'm sure it exists in other countries, although we didn't have any evidence of it," he said.

The attorney general rejected civil rights groups' charges that the Justice Department has indulged in selective enforcement of the law. He said that, except for two areas, "this administration has filed more civil rights cases . . . and voting rights cases than any previous administration."

The exceptions are school busing to achieve racial integration, which Smith called "counterproductive" compared with other methods, and affirmative action regulations, which he called "quotas."