A tidal power generating device having a large area to receive pressure of a tidal current and can be used in a water surface region. The tidal power generating device includes a cylindrical body that floats by itself and a blade that is continuously provided from a front portion to a rear portion on an outer periphery of the cylindrical body and has torsion in a single direction. The cylindrical body has an interior space through which a power generating nonrotary shaft, a connecting pipe, and a mooring nonrotary shaft pass. The power generating nonrotary shaft is provided with a fan-shaped weight and a power generator. The rear portion of the cylindrical body is sunk when the tidal power generating device is placed on water.