A power supply device including a converter having a switch circuit to which an input voltage is supplied and a coil coupled between the switch circuit and an output end from which an output voltage is output; and a control circuit comparing between a feedback voltage and a reference voltage, and on/off controls the switch circuit according to a comparison result; wherein, the control circuit includes a current gradient detection circuit performs detection of a gradient of a coil current flows thorough the coil during an off period of the switch circuit and generates a slope voltage according to a result of the detection; and an adder circuit performs one of generating the feedback voltage by adding the slope voltage to a voltage according to the output voltage and generating the reference voltage by adding the slope voltage to a standard voltage that is set according to the output voltage.