PURPOSE: To always obtain a stable performance of rinse and dewatering by a method wherein the operating time periods of an agitator in washing step and of a rotary drum in dewatering step are adjusted to adequate ones even when the frequency of commercial power source applied is changed over.

CONSTITUTION: Washing and dewatering is carried out by the rotary driving force of a motor 7 that is connected to a commercial power source 12. A control circuit 17 controls the operation time periods of an agitator 5 is washing step and of a dewatering drum 4 in dewatering step so that the ratio of operating time periods at 60Hz to that at 50Hz becomes 1:1.44 or larger. As a result, a stable performance of rinse and dewatering can be always achieved even when the frequency of commercial power source is changed over.