Aspects of the provide a control circuit of a quasi-resonant switching power supply in which the switching frequency can be adequately reduced under a light load condition. Some aspects include a control circuit of a quasi-resonant switching power supply that performs switching operation of a switching element based on a bottom detection signal. The control circuit of the switching power supply can include a dummy signal generating circuit that generates a dummy signal for replacing the bottom detection signal when the bottom detection becomes unable due to damping of a resonant waveform with decrease of switching frequency, or when the number of bottoms in the resonant waveform exceeds a predetermined times. The use of the dummy signal can increase the number of skips and thus, reduces the switching frequency.