When an operation for washing is completed and door is unlocked, water level in a water tank is monitored only for a prescribed time period. When there is no water leakage, monitoring is stopped after the prescribed time period and the power is turned off. When there is a water leakage, water level in the water tank increases. When it is detected that a first water level is reached, the door is locked to prevent opening. After a prescribed time period, the power is turned off while the door is kept locked. If the door cannot be locked, the door lock error is notified and the water collected in the water tank is drained.