A gas turbine generating apparatus according to the present invention comprises a gas turbine capable of a high-speed operation, a permanent-magnet-type generator driven at a high speed by the gas turbine, an inverter device (5) for converting alternating-current power generated by the generator into commercial alternating-current power, and a system-interconnection device (9) for interconnecting output of the inverter device to a commercial AC power supply system (10). The system-interconnection device controls the inverter device based on voltage of the commercial AC power supply system as a criterion such that output current of the inverter device is in phase with the voltage or out of phase with the voltage by a constant phase difference. A control device (18) is provided for operating the permanent-magnet-type generator (4) as a motor at a time of starting the gas turbine, and supplying alternating-current power from a direct-current power supply through the inverter device or another inverter device in addition to the inverter device to the generator for increasing a rotational speed smoothly from zero. The control device detects output current of the inverter device and controls the inverter device to keep the output current constant.