To provide a water power conversion device which enables low cost, no pollution, and large-capacity power generation by solving the disadvantages of a lift type using a drag type waterwheel. [Solution] The present invention is an underwater waterwheel, the waterwheel being a rotary waterwheel that rotates about a rotating shaft or an endless chain waterwheel in which a continuous belt is stretched between a pair of escape wheels, and the present invention is characterized in that a bottomless cup is pivotally fitted by a pivot on an outer wheel of the rotary waterwheel or on the continuous belt of the endless chain waterwheel so as to be able to stand up or lie down, in the state where the outer wheel or the continuous belt advances in the same direction as the direction of flowing water, the bottomless cup goes into a standing state, the bottom of the cup is closed by a bottom plate provided on the outer wheel or the continuous belt, and the cup functions as a water-receiving cup having high flowing water re
