There are provided a three-phase AC power supply that outputs AC voltages in LI, L2, L3 phases, photocouplers that are connected to respective lines between the L1 phase and the L2 phase, the L2 phase and the L3 phase, and the L3 phase and the L1 phase of the three-phase AC power supply, and convert respective line voltages of the L1 phase and the L2 phase, the L2 phase and the L3 phase, and the L3 phase and the L1 phase of the three-phase AC power supply into a pulse detection signal, a light receiving element of the photocoupler that is set to turn on at a predetermined threshold voltage or higher, and a controller that performs an AND operation of each pulse detection signal to generate an AND operation pulse, and determines presence or absence of an open phase and interruption and which of the L1 phase, L2 phase, and L3 phase is the open phase when the open phase is present by each pulse detection signal and the AND calculation pulse.