PURPOSE: To compensate the discharge flow of a solenoid pump in connection with the variation of temperature by using a bridge circuit formed of a resistor and a diode for temperature compensation.

CONSTITUTION: A bridge circuit is formed of resistors R11, R12 and R13, and diodes D6 and D7. The variation of resistance value RD of the diodes D6 and D7 has such a relation with that of temperature T as indicated by the characteristic line CD in the drawing. Consequently, the voltage V of an input terminal 4c varies as indicated by the characteristic line CV in the drawing. In the meanwhile, discharge fluid decreases in viscosity and weight with the rise of temperature T. Therefore, the discharge flow of a solenoid pump increases. If the resistance value of a variable resistor VR2 is fixed so that the discharge flow of the pump may become equal at the two points of reference temperature TO and TC, the variation of ambient temperature does not cause that of the discharge flow of the pump.