A hydroelectric generation system includes a fluid machine disposed in a penstock or channel, a generator driven by the fluid machine, and a control unit configured to generate a predetermined torque in the generator. Fluid flows through the penstock or channel. The penstock or channel has a main path in which the fluid machine is disposed, and a detour disposed in parallel with the main path. The detour includes an on-off valve. The on-off valve is opened when not electrified, and closed when electrified.