Provided is a power generating apparatus capable of using power generated by a heat engine (4) in combination with power of a driving source (53) provided separately from the heat engine (4). In order to prevent a problem caused when activating and stopping the apparatus, the apparatus of the present invention includes a rotary machine driving source (53) which generates a rotational driving force for a rotary machine (11) and a heat engine (4) which drives the rotary machine (11) in cooperation with the rotary machine driving source (53), wherein the heat engine includes an expander (3) which expands an evaporated working medium so as to generate a rotational driving force, the expander is provided with a bypass pipe (63) which causes a working medium inlet of the expander to communicate with a working medium outlet thereof, and the bypass pipe is provided with an on-off valve (64) which opens and closes the bypass pipe.