A vehicle-mounted power generator set including: an AC generator driven by an engine of a vehicle; an electric power conversion portion that converts an output of the AC generator into an AC output having commercial frequency; a controller having electric power conversion portion control means for controlling the conversion portion to generate an AC output having commercial frequency when a control mode is a power generation mode, and engine control means for controlling the engine to rotate at a rotational speed suitable for supplying electric power from the conversion portion to a load when the control mode is the power generation mode and controlling the engine to be suitable for driving the vehicle when the control mode is the vehicle driving mode; and a control switch inserted between each output terminal of the AC generator and each input terminal of the electric power conversion portion, and controlled to be in an ON state when the control mode is the power generation mode and in an OFF state when the control mode is the vehicle driving mode.