PURPOSE: To prevent destruction of a gate oxide film of an input MOSFET by devising the circuit so that a power voltage for a current/voltage conversion circuit comprising an operational amplifier used for detecting an overcurrent and for a comparator follows an output voltage of a semiconductor high side switch.

CONSTITUTION: A current/voltage conversion circuit 3 comprising an operational amplifier used for detecting an overcurrent and a comparator 4 are driven by a power supply supplying a voltage in following to a source potential or an emitter potential. That is, a current flows to constant voltage diodes 12, 13 from constant current sources 11, 14 and a connecting point of the constant voltage diodes 12, 13 connects to an output terminal Vout to allow output voltages V+, V- to follow the level of the semiconductor high side switch of the output terminal Vout to form the voltage V+ higher than the level by prescribed voltage and the voltage V- lower by a prescribed voltage. Thus, no excessive voltage is applied between the gate and source of an input MOSFET and destruction of a gate oxide film due to a high voltage is prevented.