PURPOSE: To change the quantity of water to be drained from a tank for washing water by draining the washing water inside the tank from a control valve until the water reaches a reference water level from a fixed water level.

CONSTITUTION: This device is provided with a control valve 50 for controlling the drainage/storage of washing water inside a tank 10 for washing water, drainage start instructing means 30 for instructing the start of drainage to the control valve 50, water level setting means 20 for variably setting the reference water level inside the tank 10 for washing water, and reference water level detecting means 40 for instructing the stop of drainage to the control valve 50 by detecting that the water level inside the tank 10 for washing water reaches the reference water level. When the drainage start instructing means 30 instructs the start of drainage, the control valve 50 starts drainage. When the water level inside the tank 10 for washing water is down to the reference water level set by the water level setting means 20, the reference water level detecting means 40 instructs the stop of drainage, and the control valve 50 stops drainage. Thus, water resources can be effectively utilized without preparing tanks for washing water with the different values of drainage discharge quantity.