PURPOSE: To reduce a current capacitance required by a diode which protects insulated-gate control transistors, including insulated-gate bipolar transistors and field effect transistors, from an overvoltage by a dynamic clamping method, and to reduce the size of the diode.

CONSTITUTION: A protective diode 30 having an avalanche breakdown voltage which is lower than a withstand voltage of an insulated-gate control transistor 10 is connected between a gate and a main terminal of the transistor 10. For example, a depletion type bipolar transistor 41 is connected, as a nonlinear element 40 which has a current saturation characteristic, between the gate and a drive circuit 20. When a voltage Vc applied to the transistor 10 exceeds the withstand voltage of the transistor 10, the diode 30 causes breakdown. An avalanche current Id, which flows into the diode 30 while the transistor 10 is temporarily turned on and an overvoltage is absorbed, is limited to a saturation current value of the non-linear element 40 by means of the non-linear element 40, so that the avalanche current is reduced to some fraction of one when compared with an avalanche current fl-owing through a conventional protection circuit.