PURPOSE: To reduce the switching loss extremely to ensure a high efficiency by applying the fixed reverse bias voltage to the input electrode of the switching element when the switching element is turned off.

CONSTITUTION: The pulse given from the pulse generator is applied to the base of transistor Q6 to turn on Q6 with Q5 and Q4 turned on and off respectively. While if Q6 is turned off, Q4 and Q5 are turned on and off respectively. When Q4 is turned on, Q2 is also turned on to turn on Q1 and then turned off when Q5 is turned on to turn off Q1. In this case, Q3 is turned off when Q4, Q2 and Q1 are turned on; while the reverse bias voltage due to DC power source E2 is applied to the base of Q1 when Q5 is turned on with Q2 and Q1 are turned off. As a result, the turn-off time is reduced for Q1, and thus the switching loss can be decreased sufficiently to realize a high efficiency.