PURPOSE: To make operation accurate by securely detecting an overcurrent for protecting the semiconductor device and stabilizing the potential of a control terminal to a reference potential point.

CONSTITUTION: The source of a MOS transistor(TR) as the semiconductor device 10 is connected directly to a ground potential point as the reference potential point E of operation to stabilize the potential at a control terminal Tc. Then a parallel circuit of a diode 20 and a light emitting diode 30 is connected to the drain side of the semiconductor device 10 in series and a current I is shunt to the both, thereby setting a detected value of the overcurrent at, for example, their shunt ratio. A phototransistor is connected as a light receiving element 40 between the control terminal Tc and source and optically coupled with the light emitting diode 30. Consequently, when the overcurrent is detected through the light emission of the light emitting diode 30, the light receiving element 40 is placed in operation with the emitted light L to turn OFF, for example, the semiconductor device 10, thereby eliminating the overcurrent state.