PURPOSE:To improve the operation of a solenoid valve in liquid, by applying a rated voltage to the solenoid of the solenoid valve by a control device which is provided with a relay circuit, a constant voltage circuit, and a pulse circuit only when the solenoid valve begins to open, and applying only a low voltage required to keep the solenoid valve at the opened state. CONSTITUTION:A signal, which turns ON and OFF a solenoid of a solenoid valve 6, is inputted to both a solid state relay circuit 4, which performs flip flop operation, and a pulse circuit 3, from a control device. When the signal is inputted to the pulse circuit 3, a light emitting element emits light, and a voltage pulse corresponding to the light from the light emitting element is applied to the solenoid 6 from the output side including a light receiving element. The relay circuit 4 continues to supply the constant voltage from a constant voltage circuit 5 from the time when the circuit 4 is turned ON by the first signal to the time which it is turned OFF by the next end signal. As a result, the voltage shown by a solid line is applied to the solenoid 6 of the solenoid valve during the time the relay circuit is ON. Said voltage is different from the conventional voltage shown by a chain line. The voltage for a short time at the beginning corresponds to the rated voltages of the solenoid but the voltage for the rest of the time is considerably lower than the rated voltage.