PURPOSE: To reduce the rising factor of a current flowing into a snubber circuit in a switch-off mode and also to decrease the noises generated in a switching mode, by adding a reactor and a diode to a transistor which performs a switching action.

CONSTITUTION: A reactor 5 is connected in series to the emitter of a transistor TR2, and a diode 6 is connected between a terminal which is not connected to the emitter of the reactor 5 and the base of the TR2. It is also available to connect plural diodes 6 in series instead of just a single unit. The inserting direction of the diode 6 is decided so that the electromotive force of the reactor 5 which is produced in response to the reduction of current when the TR2 is off is supplied to the base of the TR2 through the diode 6. As a result, the rising factor is reduced for the current which flows into a snubber circuit 3 in switch- off mode. Furthermore, the noise produced in a switching mode is also reduced.