A power supply device that includes a switch circuit to which an input voltage is supplied, a coil coupled between the switch circuit and an output terminal from which an output voltage is outputted. A voltage adding circuit adds a slope voltage to a reference voltage. A control unit compares a feedback voltage corresponding to the output voltage and the reference voltage and switches the switch circuit at a timing corresponding to a comparison result of the feedback voltage and the reference voltage. A slope adjustment circuit differentiates a current flowing in the coil and adjusts a slope amount of the slope based on a differentiation result of the current.