To lower a rotational speed of a turbine rotating by inertia as rapidly as possible.

This power generation system includes: a main generator 10 that generates electric power to be transmitted to a power system; an auxiliary generator 40 that generates electric power to be transmitted to an in-station power circuit; a rotating shaft 30 that is connected to the main generator 10 and the auxiliary generator 40; and a water wheel 20 for power generation as a turbine for rotating the rotating shaft 30. The main generator 10 and the auxiliary generator 40 generate electric power by the rotation of the water wheel 20 for power generation. The auxiliary generator 40, upon stopping of energy supply for rotating the water wheel 20 for power generation and stopping of transmission from the main generator 10 to the power system, transmits electric power generated by the inertia of the water wheel 20 for power generation to the in-station power circuit to regeneratively brake the rotation of the water wheel 20 for power generation.