PURPOSE: To effect extremely quick soaking of washed rice grains with about 30% of water in proper quantities, by immersing washed rice grains in water, evacuating the system to remove gases from the inner part of the grains and returning to atmospheric pressure.

CONSTITUTION: Washed rice grains are charged into a basket 1 having a number of water-passing holes on the circumferential wall and the basket is put into a vacuum pan 3. Water is supplied to the vacuum pan 3 through water-feeding and discharging means 14W17 to immerse the washed rice grains in water. The valve 17 of the water-feeding and discharging means 14W17 is closed and the valve 11 of air-feeding and exhausting means 10, 13 is opened to evacuate the pan 3. After maintaining the pan in evacuated state for about 10min, the valve 11 of the air-feeding and exhausting means 10, 13 is opened to return the system to atmospheric pressure.