A direct-current voltage is applied to a series circuit composed of a switching transistor, a sense resistor, and a coil. A control circuit is configured to be capable of performing current control in which the control circuit, after turning on the switching transistor, determines a turn-off time point of the switching transistor based on a sense voltage appearing across the sense resistor, and to turn off the switching transistor during the current control if, despite a predetermined time having passed after the switching transistor being turned on, the sense voltage does not reach a predetermined threshold voltage.