To easily detect disconnection and poor contact of at least one phase between a vector control inverter and a motor.

An absolute value operation part 6A detects an absolute value of deviation of a Q-shaft current output from a Q-shaft current command in vector control. A disconnection judging part 6B judges disconnection or poor contact of at least one phase of the inverter output when the absolute value of the deviation exceeds a specific value and this state continued over set times in a certain period. Instead of the absolute value of the deviation of the Q-shaft current, judging the disconnection and poor contact from the absolute value detection of a D-shaft current and integrating the absolute value of the deviation using a first delay filter and judging by comparing the voltage from the integrated value are included.