In many Asian countries, a bumper crop of rice is cause for national jubilation. Japan had one this fall, following four consecutive bad years in the fields, but it is hard to find anyone who is particularly happy about it.

"We have no special festivities planned," said Tomio Yumoto, chief of a farmers' association in this pleasant, well swept town 25 miles north of Tokyo. "Any celebrating can wait until our regular festival on Dec. 23."

Times have changed for Japan's farmers. In the four decades that Yumoto has worked his land, the country has evolved one of the most byzantine and tightly policed agricultural economies in the world. Consider the following:

* Plentiful rice this year will not mean lower prices for consumers. Before the first grain was harvested, farmers and government officials sat down to fix the price. They kept it in the vicinity of four times the price of rice in the United States.

* The crop will not mean a big surge in rural income. Most Japanese farmers, like Yumoto, till the soil only part-time, for pocket money or personal satisfaction. Jobs in nearby factories and offices account for about 80 percent of their income.

* The crop will not mean savings for taxpayers. The central government each year is committed to buy half the crop, and this time with oversupply will be able to resell less of it. It will have to find money somewhere to store the surplus.

Japan's rice support system is fodder for argument in the press and government here. It is scourged for ignoring laws of supply and demand, protecting gross inefficiency in production and costing Japanese taxpayers billions of dollars a year.

But even its critics concede it has served an important social function. It has helped save a way of life that is a key to Japanese civilization. And it has helped spread the prosperity generated in industrial Japan to the rest of the country.

In fact, surveys now show that the nation's 4.6 million farming households, which contain about 10 percent of Japan's work force, have slightly higher incomes, greater floor space and more TV sets, cameras and lounge furniture suites than do their city counterparts.

Japanese shoppers are generally not aware that the prices they pay for rice are absurd by the rest of Asia's standards. Those who are tend to feel it is a duty to help out country folk, who are still viewed as the underprivileged.

"Many city workers still have roots in the countryside," said a rice specialist at the Central Union of Agricultural Cooperatives in Tokyo. ". . . They have sympathy."

Its benefit or harm aside, the rice policy is an immovable feature of Japan's political landscape. The ruling Liberal Democratic Party put it in place and knows that farmers have constitituted one of its most important and loyal voting blocs.

Japanese don't like to admit it, but the rice economy is based on unbridled protectionism. Rice imports are banned outright. For many years wild rice, an unrelated plant species, was also banned, due to fears it would open the door to the real thing.

Protection is needed, the argument goes, because Japan is small and its farms tiny, averaging about three acres. They can never compete with the mechanized fields of Louisiana or the paddies of Thailand, where much of the world's export rice is grown.

The destruction of Japanese farming would also jeopardize national security, it is said. "We hold to the principle that food must be supplied by oneself to the greatest extent possible," Yumoto said.

When Americans complain about the coddled standing of rice here, the Japanese like to point out that they are the United States' largest customer for other farm products, such as corn, wheat and soybeans. Last year, purchases of U.S. farm products hit $6.2 billion, almost three times the volume bought by the next biggest customer, the Netherlands.

Before World War II, Ageo was a quiet country town where 90 percent of the people farmed. Today, it has grown into a city of 175,000, a bedroom community for Tokyo, a 45-minute train ride away. But with the help of government, farmers hang on everywhere.

Even in the city center, almost every patch of ground without a building shows their handiwork. Rows of closely tended corn stand between middle-class homes; the dry stubble of harvested rice shows next to a filling station; onions grow by the parking lot of a smart restaurant.

Yoshiaki Amanuma is typical of the modern farmer. On most days, he is found in an Ageo city government office, where the work is clean and well paying enough for him to appear with an Yves St. Laurent clasp on his tie.

But at special times, such as harvest or the arrival of government inspectors, he takes a few days off and dons work clothes on a three-acre farm several miles outside town that he and his 67-year-old father, Shigeru, maintain.

"Farming is not chic among young people today," Amanuma noted. But he was raised with the understanding that he would one day take over the family property, where a solid, thatch-roofed farmhouse contains a color television set.

The government's near dictatorial control dates to an emergency wartime act in 1942. The supports and incentives in place today rival the United States' in complexity and cost about $4 billion a year, accounting for much of the national debt.

To give farmers a piece of the postwar prosperity, government began marking up the prices it paid for rice in the 1950s. This encouraged more and more farmers to grow it, until a state of chronic oversupply was reached in the late 1960s.

In 1969, the government shifted gears. Borrowing a device learned from the United States, it began paying farmers to make kyukoden, literally sleeping or fallow fields. These programs were followed by incentives to plant crops other than rice.

In the 1970s, the glut was so bad that the government unloaded old stocks overseas, earning only a fraction of what it had paid for it but happy to get something. In 1980, Japanese exports accounted for 5 percent of all world trade, driving prices down and provoking angry protests from the United States.

Then glut quickly turned to shortage. Four consecutive bad harvests since 1980 and spoilage caused by faulty use of preservatives in storage facilities had exhausted the nation's reserves by early this year.

Fending off opposition charges of mismanagement, the government brought in rice from South Korea. Farmers screamed in protest. Officials responded that it was not an import at all, just repayment of a "rice loan" to South Korea several years ago.

The big crop of 1984 might have been nothing but headaches for officials here were it not for the empty storage houses. About 900,000 tons of the 12 million-ton crop will go there, with the objective of creating a 1.5 million-ton reserve, less than a two-month supply.

It is difficult to overstate the importance of rice in the evolution of Japanese civilization. It has long been inseparable from food in general. The same word, gohan, is used in Japanese for both.

To the dismay of older people, rice has been going out of style on Japanese dinner tables since the early 1960s. That which is still eaten is known more and more by the imported English word ricu. Price and convenience seem to be behind the decline.

Even with the electric cooker that is now a standard appliance in Japanese kitchens, making rice requires thought and preparation, Yumoto notes with approval. "It is an important element of family life. It encourages members to eat together and to talk to one another."

The government's long-term goal is to reduce rice production further. This year a quarter of Japan's rice fields will be put to other use. The goal is to put supply in line with demand and increase production of wheat, which Japan buys in enormous quantities from the United States. Still, people like Yumoto, raised on rice cultivation, are loath to make the switch. That is one more reason why the harvest of 1984 was not good news here. It is sure to bring new pressure and "guidance" from government agricultural officials. "In the old days, a good harvest meant unrestrained joy," Yumoto said. "Now it means only that next year our space will be reduced again."