PURPOSE: To enable high-speed operation with low energy consumption by driving a P channel power MOS transistor by a buffer and stabilizing the power source voltage of the buffer by a constant voltage circuit.

CONSTITUTION: A control signal to be inputted from a control terminal 4 is inputted to an inverter 11, which is a CMOS buffer, and the output of the inverter 11 is supplied to the gate of a P channel MOSFET 3. The power source voltage of the inverter 11 is determined by the constant voltage circuit, which is composed of a Zener diode 12 and resistor 13, and even when the power source voltage of a switching circuit (the voltage of a terminal 1) is fluctuated, the power source voltage of the inverter 11 is not fluctuated. Accordingly, a drive voltage is not fluctuated as well and the power MOSFET 3 can be always stably driven. Then, a switching speed is determined by the speed of the inverter 11. Thus, operation can correspond to the high speed with the low energy consumption.