A power supply device includes a converter and an arithmetic circuit. The arithmetic circuit calculates a present control command value at predetermined intervals based on a digital value, which reflects an output voltage of a converter, from a conversion circuit. The arithmetic circuit calculates the difference value between the present control command value and a previous control command value. Based on the difference value, it is determined whether a signal output circuit outputs a high-level signal toward a charge terminal or a low-level signal toward a discharge terminal. As a result, the number of output lines corresponds to at least the number of charge terminals and discharge terminals. The above features can be achieved with a simple circuit configuration by adding a discharge and charge circuit and without increasing the number of output lines.