In controlling switching elements of a current source inverter, a switching loss in the switching element is prevented according to a normal switching operation for a commutation operation, without requiring any particular control.In the commutation operation of the current source inverter, a timing for driving the switching elements is controlled in such a manner that an overlap period is generated, during when both a switching element at the commutation source and a switching element at the commutation target are set to be the ON state, a resonant circuit is controlled based on the control of the switching elements having this overlap period, and resonant current of the resonant circuit reduces the switching loss upon commutation operation of the switching elements. By controlling the generation of the resonant current of the resonant circuit by using the control of the switching elements having the overlap period, the resonant current generated by this control renders current and voltage of the switching element at the commutation source to zero when commutation is performed, thereby reducing the switching loss in the commutation operation.