PURPOSE: To easily detect the state of each phase of a power supply, by setting a part or the whole of the supply current of the rectifying arm of a rectifying circuit for rectifying the voltage of a multiphase AC power supply to the input current of a photocoupler and discriminating the open phase of the AC power supply on the basis of the disappearance of the output signal of the photocoupler.

CONSTITUTION: For example, when it is supposed that a three-phase power supply line P1 generates an open phase, only the single phase connection of three-phase power supply lines P2, P3 is formed and the DC power supplies obtained from positive and negative rectifying circuits 2P, 2N are not different from those at a usual time. Since no current flows to the line P1, that is, the rectifying arm corresponding to P12, that is, a diode D11 or D12, the photodiode PD1 in a photocoupler PC1 becomes a non-lighting state and the phototransistor PTr 1 in the photocoupler PC1 is not operated and the open phase state of the three-phase power supply line P1 is informed to the port (a) of an MPU 5 through an open phase detection signal 8a and an open phase alarm apparatus 6 is operated. By this method, the state of each phase of the power supply can be easily detected.