When a DC/DC converter switches between a primary voltage control mode, a secondary voltage control mode, and a current limiting mode as operation modes, an I-term resetting processor outputs an I term depending on the duty ratio in the operation mode before being switched to a PID processor. The PID processor performs a PID control process based on the input I term. A drive duty ratio setter outputs a drive duty ratio immediately after the control mode switching, which is substantially equal to the duty ratio in the operation mode before being switched. As a result, the duty ratios in the operation mode before being switched and the operation mode after being switched are made continuous with respect to each other.