PURPOSE: To achieve higher washing force in a prewashing process by raising the temperature of a washing water in the prewashing process upto a range lower than the denaturation temperature of protein.

CONSTITUTION: A water supply valve 7 is opened to supply water and when a washing water builds up by a specified amount on the bottom of a washing tank 1, the washing water is jetted out of a washing nozzle 4 to wash dishes 2. Then, the washing water is again accumulated on the bottom of the washing tank 1. This operation is repeated. When the temperature of the washing water rises up to a temperature range lower than the denaturation temperature of protein. The rewashing process ends with a temperature controller 9 and the washing water is discharged out of the washing tank 1. This mechanism allows the swelling of the fouling by starch attributed to grains of rice and the washing before the fouling by protein attributed to the beginning of egg coagulation thereby achieving higher washing force in the prewashig process.