In a power supply device including a PWM-controlled DC/DC converter which changes an output voltage on the basis of a setting signal, and a control unit, the DC/DC converter includes a circuit for changing a dead-time control voltage in accordance with a control signal. A dead-time control circuit is arranged, which sets the ON duty of a PWM main switching element at a predetermined time constant in accordance with the dead-time control voltage. The control unit outputs the setting signal and control signal. In this arrangement, an input rush current generated in changing the output voltage during operation can be reduced, and the time required for causing the output voltage to reach the setting value can also be shortened.