A semiconductor device for controlling a switching power supply is provided, which can reduce power consumption at a light load of the switching power supply and improve the efficiency of the power supply. During the intermittent switching operation of a switching element, when a return signal is outputted from a light load detection circuit within the set time of a transformer reset detection time setting circuit during intermittent stop, the switching operation of the switching element is restarted in response to a transformer reset signal from the transformer reset detection circuit after the return signal is outputted. When the return signal is outputted from the light load detection circuit during the intermittent stop after the set time of the transformer reset detection time setting circuit, the switching operation of the switching element is restarted regardless of the transformer reset signal only in response to the output of the return signal.