A power supply device includes a convertor and an arithmetic circuit. The convertor supplies electric power by performing an ON and OFF operation of a switching element. The arithmetic circuit calculates a control command value at predetermined intervals based on a digital value, which reflects an output voltage of the convertor, from a conversion circuit. Based on the control command value, a pulse signal generation circuit determines a duty ratio of a pulse signal to make the output voltage of the convertor stable. A driving signal with a finely adjusted duty ratio can be generated by an operation clock signal having a lower frequency than a conventional device. As a result, the number of output lines is appropriate if there is at least a single output line for the pulse signal from the pulse signal generation circuit.