A water flow power generator includes a nacelle, a vane wheel that is disposed so as to be rotatable relative to the nacelle, and that is rotated by a water flow while including a plurality of blades, a power generator that is disposed inside the nacelle, and that generates electric power by using rotating power transmitted from the vane wheel, and a vane wheel rotation stopping mechanism that is disposed in the nacelle, that includes a rod which can enter the inside of a rotational trajectory of the vane wheel, and that stops the rotation of the vane wheel.