To eliminate the need to set complicated parameters only by setting only an upper and a lower limit value as target values of water level and perform water level control at low cost with good response.

A water level sensor 2, a drainage pump 3, and a controller 5 are provided and the discharging capability of the drainage pump 3 installed in a water tank 1 is so controlled by the controller 5 that the discharge of the drainage pump 3 is proportional to the water level in the water tank 1 detected by the water level sensor 2. At this time, an arbitrary upper-limit set value of the water level in the water tank 1 is made to correspond to 100% of the discharging capability of the water drainage pump 3 and an arbitrary water level below the upper-limit set value of the water tank 1 is made to correspond to 0% of the discharging capability of the water drainage pump 3; and the 100% of the discharging capability is regarded as the upper end of the control range and discharging capability corresponding to the minimum discharge amount of the water drainage pump 3 is regarded as the lower limit of the control range.