When the three U-, V-, W-phase arms of a DC/DC converter are turned on, they are alternately turned on by gate drive signals. When the U-, V-, W-phase arms are alternately turned on, an upper arm switching device of the U-phase arm, for example, is turned on, and thereafter a lower arm switching device of the U-phase arm is turned on. Thereafter, an upper arm switching device of the V-phase arm which is next to the U-phase arm is turned on, and thereafter a lower arm switching device of the V-phase arm is turned on. The upper and lower arm switching devices are thus turned on in a rotation switching process.