An integrated circuit for a power supply circuit that generates an output voltage at a target level from an AC voltage input thereto. The power supply circuit includes an inductor, and a transistor that controls a current flowing through the inductor. The integrated circuit drives the transistor based on the inductor current and the output voltage. The integrated circuit has a signal output circuit including a first comparator circuit comparing the inductor current and a predetermined current value, a timer circuit measuring a time elapsed since the value of the inductor current becomes smaller than the predetermined current value, and a second comparator circuit comparing the output voltage and a lower-than-target level. The signal output circuit indicates that the AC voltage is not input to a rectifier circuit, when the elapsed time reaches a predetermined time period and a level of the output voltage is lower than the lower-than-target level.