PURPOSE: To prevent the occurrence of hunting operation during a low inflow amount by determining the water level rise speed of a pump well and prevent the occurrence of flooding to a pump station owing to the rapid increase of an inflow amount by lowering a starting water level when a rise speed is high and raising the starting water level when it is low.

CONSTITUTION: Drain, such as rainwater, flowing in a floodway is guided to the pump well 7 of a pump station through a drainage main pipe 5 and pumped up and discharged with the aid of a plurality of drainage pumps P (P1-P3). A water level sensor 21 is arranged to the pump well 7, when the water level is raised to a starting water level, the drainage pump P is started, and when it is lowered to a stop water level, the pump is stopped. In this case, in a water level change speed computing device 22, the change speed V (a water level rise speed or a water level lowering speed) of a pump water level (h) is computed. Based on the computed result, a starting water level and a stop water level are set by a starting water level varying means 23 and a stop water level varying means 24. Based on the starting water level and stop water level, the starting and the stop of each pump P are controlled by a command means 26.