To provide a power supply which has prevented drop in the efficiency by a reactor.

When a zero-cross point of AC voltage is detected by a control circuit 15, a switching element 13 is turned on for a constant period, an Ac power supply 1 is terminated via a reactor 3, thereby energy is accumulated in the reactor 3. After the termination, the energy accumulated in the reactor 3 is started to be released. Thereafter, when the power supply voltage exceeds the voltage across a capacitor 7, a current from the AC power supply 1 flows through a rectifying circuit 5 passing a bypass line 17 and therefore the efficiency is not lowered due to loss of copper of the reactor 3.