PURPOSE: To improve the reliability of an apparatus by adding another six-pulse inverter to two six-pulse inverters constituting the main circuit of a twelve-pulse inverter apparatus.

CONSTITUTION: When two inverters or more out of six-pulse inverters 2-4 are normal, an inverter apparatus uses two normal six-pulse inverters (e.g., 2, 3) and twelve-pulse operates a synchronous motor 5. In this case, switches 6-9 are OFF and those 10-12 are ON. Also, a rectifier phase signal outputted from a twelve-pulse inverter control circuit 16 is transmitted to the rectifier of each of the six-pulse inverters 2, 3 via a rectifier phase signal switching circuit 14, and a power inverter phase signal is simultaneously transmitted to the power inverter of each of six-pulse inverters 2, 3 via the switching circuit 15 of the signal. When a failure disabling the continuation of operation occurs, the inverter apparatus is stopped once and a failure signal is outputted to a failure-state decision circuit 17 and displayed by an indicating circuit 18.