To provide a power supply which has no overshoot in the output even if there is an interruption of AC input which continues for as short a time as a control system does not stop.

The ON/OFF state of the AC supply voltage input from input terminals 1, 2 is detected by an AC voltage detection circuit 11. When the AC voltage detection circuit 11 detects that a commercial AC power supply is cut off, it resets a soft start circuit 10 after a given period of time. Next time the AC power supply is turned on, a soft start signal is output from the soft start circuit 10 to start up a power circuit by soft start. Thereby, overshoots which can be generated by instantaneous cutoff of AC input can be reduced.