PURPOSE: To prevent a FET element from being destructed due to the FET source potential fluctuation at turn-on, turn-off by making the ground of a FET driving circuit equipotential to the source of the FET and isolating the power supply from ground.

CONSTITUTION: A transistor TR1 is operated by an input signal and connected to ground, then a transistor(TR)2 is operated. A current starts flowing to a field effect TR FET and a voltage of an inductance L is generated. The collector current of the TR2 is increased and the TR2 transits from the active region to the saturation region. The gate voltage of the field effect TR FET reaches the voltage VCC charged in the capacitor C1. The current in the field effect TR FET is made constant and the voltage of the inductance L is made constant and stable.