A control circuit for an AC/DC converter includes an AC detection circuit that periodically determines from a change in the AC input voltage whether AC voltage is being input and, upon determining that AC voltage is being input, outputs an AC detection signal that takes a HIGH level only for a prescribed duration; and an enable signal conversion circuit that filters the AC detection signal to generate the enable signal that is in a HIGH state when the AC detection signal is in the HIGH level and that becomes the LOW state after a prescribed time has passed since the AC detection signal becomes a LOW level unless the AC detection signal rises to the HIGH level again, thereby producing the enable signal that is more responsive.