A pair of upper and lower semiconductor switching elements can switch between the conductive state and the non-conductive state when control voltages vary. A controller controls the levels of the control voltages to alternately turn on the upper and lower semiconductor switching elements. The controller controls the absolute value of the second control voltage of the lower semiconductor switching element so as to reach a mean voltage before and after the time of transition between the conductive state and the non-conductive state of the upper semiconductor switching element. The mean voltage is lower than the absolute value of a threshold voltage and higher than a reference voltage.