TOKYO (AP) _ Japan's major agricultural unions vowed to fight to preserve Japan's farm import barriers, but agreed to the first cut in the price paid for rice in 31 years, a union official said.

Rice, the nation's staple food, is about three times as expensive in Japan as in the United States in spite of government subsidies, because of an import ban.

In a statement to be presented to the government, Zenchu, the Central Union of Agricultural Cooperatives, demanded that the present food control system be preserved for the sake of food security.

''To assure the security and stability of the food supply, we oppose any opening of the rice market, and will continue our opposition to lifting import barriers for beef, oranges'' and other agricultural products, the statement said.

Representatives of farm cooperatives throughout Japan also agreed to request that the average price of rice be cut by about 3.37 percent. Increasing yields and lower labor costs contributed to lower costs in the past year, justifying a price cut, according to Zenchu's written statement.

The government has asked that the price it pays producers for rice be cut by 7 to 8 percent this year.

Japan buys almost all rice from farmers and sells it to consumers at somewhat lower prices.

The U.S. government and American farmers have criticized Japan for restricting imports of rice and other farm products.