PURPOSE: To enable to obtain a rated discharge quantity of liquid from an electromagnetic pump at all times, by connecting a Zener diode to a constant circuit, and changing the duration for supplying current to the electromagnetic pump according to the change in the supply voltage of the electromagnetic pump.

CONSTITUTION: A Zener diode 3 is connected to a time-constant circuit 2 which determines the width of pulses produced by a monostable multivibrator. When the supply voltage E of an electromagnetic pump 1 is changed from E1 to E2 (E1<E2), the width of pulses produced by the monostable multivibrator is reduced automatically, and the discharge quantity of the pump 1 is kept at a value substantially equal to the rated discharge quantity of the pump 1 at the time when the supply voltage is E1. Here, the discharge quantity of the pump 1 is selected in a desired manner by changing the frequency of trigger pulses applied to the base of a transistor Tr1 of the monostable multivibrator.