An output voltage detection circuit is provided for accurately detecting a secondary side output voltage based on a voltage induced to an auxiliary winding in an isolated switching power supply including a transformer having the auxiliary winding. A voltage waveform processing circuit (10-1) processes the waveform of the voltage induced to the auxiliary winding (7-3), a hold circuit (10-2) holds the voltage peak value after the processing, a secondary side current detection circuit (24) detects a period during which current passes through a secondary side winding (7-2), a correction circuit (22) performs correction to decrease the peak voltage which has been held by the hold circuit (10-2) during the period, and the corrected voltage is fed back to a control circuit (9). Meanwhile, a reset circuit (23) resets, for a fixed period after the current passes through the secondary side winding (7-2), the peak voltage having been held by the hold circuit (10-2).