PURPOSE: To improve reliability, to simplify, and to reduce a cost by a method wherein the starting timing of a pump is constantly properly detected and the use of a water level detecting means is eliminated.

CONSTITUTION: A pressure in a position situated upper stream from the location position of a delivery valve 7 in a delivery pipe 6 is detected by a pressure sensor 12 and after a detected pressure is converted into an electric signal by a pressure converter 13, the signal is inputted to a comparing circuit 9A. Meanwhile, a pressure in the delivery pipe 6 when a water level in a suction water well 1 attains a pumping-up starting water level WL is converted into an electric signal value, which forms a set value. The set value is previously inputted from a setter 9B to a comparing circuit 9A. An electric signal value inputted from the pressure converter 13 is compared with a set value inputted from the setter 9B by the comparing circuit 9A. At a point of time when an electric signal value inputted from the pressure converter 13 attains a value equal to a set value, namely, at a point of time when a water level in the water suction well 1 attains the pumping-up starting water level WL, a starting signal is outputted to a primer mover 2 for driving a pump or a clutch 10 to start the pump.