PURPOSE: To eliminate a need to set a plurality of stages of water levels, corresponding to the number of pumps, at a suction water tank and to reduce the depth of the suction water tank by a method wherein from an inflow rate to the suction water tank and an estimated flow rate of drain from the suction water tank, the necessary number of working pumps is decided.

CONSTITUTION: The delivery lines 13 of a plurality of drain pumps 11 are connected to a delivery water tank 14 formed in a ground. In control of operation of the drain pump 11, from an inflow rate of a suction water tank 5 and an estimated flow rate of drain from the suction water tank 5, the necessary number N of working pumps is decided. When a water level in the suction water tank 5 attains a starting water level, the number N of the pumps 11 is started and brought into a waiting operation state. When a water level in the suction water tank 5 exceeds a drain starting water level, drainage operation is started to effect control so that a water level in the suction water tank 5 is kept at a constant value. When the number of the working pumps exceeds the necessary number N of the working pumps, the pumps 11 are switched to a waiting operation state. When the waiting operation state is continued for a given time or more, operation of the pumps 11 is stopped.