PURPOSE: To reduce the ON resistance of a FET and to increase the gate voltage by applying a voltage generated from 1st and 2nd photovoltaic elements coupled optically with 1st and 2nd light emitting elements to a gate of a MOSFET.

CONSTITUTION: When a control signal is outputted from a control power supply 21, an LED 31 is stimulated and an LED 33 coupled optically thereto generates an electromotive force. The electromotive force is applied as a gate voltage of an MOSFET 35, the FET 35 is turned on and a load current IL flows. The LED 32 is lighted by the current IL and an electromotive force is generated from the LED 34 receiving it. Thus, the gate voltage of the FET 35 is increased further and acts on reducing the ON resistance. Thus, a larger luminous quantity is obtained more than the case of driving the LED by the control power supply only and the ON resistance of the FET as the switch element is reduced.