The present invention is directed to decrease temperature rise in a power supply circuit in a standby mode by using existing signals and circuits while suppressing cost. In a power supply circuit having a power-factor improving circuit for improving a power factor seen from an AC power supply side, and a switching regulator constructed by a self-oscillating converter to which the power-factor improving circuit is connected on a primary side of a transformer and from which DC voltages for a load are output on a secondary side, a lamp turn-on permission signal to be supplied to a lamp driving circuit is detected by a signal detecting circuit. In a standby mode in which the lamp turn-on permission signal is not detected, operation of a PFC (Power Factor Correction) control unit provided in the power-factor improving circuit is stopped.