An inverter apparatus for polyphase AC motor drive comprising: an inverter circuit connected between a power source and a polyphase AC motor configured to convert power from the power source to polyphase AC power to be supplied to the polyphase AC motor; a first power supply switching device provided between the power source and the inverter circuit configured to open/close the electric path between the power source and the inverter circuit; a motor relay switching device provided between an output point of the inverter circuit for each phase and the polyphase AC motor configured to open/close the electric path between the output point and the polyphase AC motor; and a control circuit configured to perform switching control of the first power supply switching device, the motor relay switching device and the inverter circuit, characterized in that: a second power supply switching device (35U,37U) provided for each phase in the inverter circuit on either an upstream side of a switching device of an upper arm circuit or a downstream side of a switching device of a lower arm circuit of the inverter circuit; wherein a first diode is provided, connected in parallel with the first power supply switching device and in the forward direction with respect to the current supply direction, and a second diode is provided, connected in parallel with the second power supply switching device (35U,37U) and in the reverse direction with respect to the current supply direction. an inverter circuit connected between a power source and a polyphase AC motor configured to convert power from the power source to polyphase AC power to be supplied to the polyphase AC motor; a first power supply switching device provided between the power source and the inverter circuit configured to open/close the electric path between the power source and the inverter circuit; a motor relay switching device provided between an output point of the inverter circuit for each phase and the polyphase AC motor configured to open/close the electric path between the output point and the polyphase AC motor; and a control circuit configured to perform switching control of the first power supply switching device, the motor relay switching device and the inverter circuit, characterized in that: a second power supply switching device (35U,37U) provided for each phase in the inverter circuit on either an upstream side of a switching device of an upper arm circuit or a downstream side of a switching device of a lower arm circuit of the inverter circuit; wherein a first diode is provided, connected in parallel with the first power supply switching device and in the forward direction with respect to the current supply direction, and a second diode is provided, connected in parallel with the second power supply switching device (35U,37U) and in the reverse direction with respect to the current supply direction. a second power supply switching device (35U,37U) provided for each phase in the inverter circuit on either an upstream side of a switching device of an upper arm circuit or a downstream side of a switching device of a lower arm circuit of the inverter circuit; wherein a first diode is provided, connected in parallel with the first power supply switching device and in the forward direction with respect to the current supply direction, and a second diode is provided, connected in parallel with the second power supply switching device (35U,37U) and in the reverse direction with respect to the current supply direction.