PURPOSE: To provide an AC-DC converter which never fails and has a small thermal loss.

CONSTITUTION: A rectifier circuit 13 and two rectifier capacitors 141, 142 connected in series on the DC side of the rectifier circuit 13 are provided. Moreover, a relay having a contact point 15a connected to the neutral point A of the two rectifier capacitors 141, 142 and to one of the AC terminals, a voltage detecting circuit 30 which detects whether the potential at the neutral point A is 100V or 200V and generates a signal representing the result, a relay control circuit 40 which receives the output signal from the voltage detecting circuit 30 to control the voltage to be applied to a coil 15a (for DC 100V) of the relay, and a constant voltage and constant current circuit 50 of a switching control system are also provided. When the potential at the neutral point A is determined to be 100V, the relay contact 15 is turned on and when it is 200V, the relay contact 15 is turned off.