PURPOSE: To improve pump efficiency and automate pump operation in case of emergency by providing a motor speed control means, changing-over the number of pump revolutions in three stages to control it, and performing sequential operation to the length of submerged part of a pump and a water level rising rate.

CONSTITUTION: A motor speed control circuit composed of a water level gauge 7, detector 8, computing element 9, controller 10, and change gear 11 is provided. The water level gauge 7 transmits the water level fluctuation of a lower level A to the detector 8, which inputs water level signals and water level rising signals in the computing element 9. The control program for changing-over the number of pump revolutions in three stages to control it according to the length of the submerged part of the pump is entered beforehand in the computing element 9. And a revolution is selected according to the length of the submerged part and water level rising speed, and the change gear 11 is controlled by the controller 10 so as to be adapted to the number of revolutions. Thus, the pump efficiency can be improved and the pump operation in case of emergency can be automated by making it possible to change-over the number of pump revolutions in three stags to control it and performing sequential operation to the submerged amount of the pump and a water level rising rate.