A water flow electricity generating device includes: an annular stator in which a coil is provided; a permanent magnet that causes a magnetic force to act on the coil; a rotor main body that is disposed on an inner circumferential side of a stator core to support the permanent magnet; a blade support unit that is provided on a front side of the rotor main body; a rotor that includes a rotating blade projecting radially outward from the blade support unit; and water-lubricated bearings that are provided in the stator while being opposed to the rotor main body, the water-lubricated bearings supporting loads in a thrust direction and a radial direction. The blade support unit is disposed on a front side of the rotor main body, and the rotor includes a float unit that is provided on the front side of the rotor main body.