PURPOSE: To improve the efficiency of a water turbine, by rotatably attaching water turbine blades to radial spokes which connect a horizontal rotary shaft and a ring shaped frame so that the blades are opened only to the side of an upstream, and alleviating and removing the resistance against the rotation of the water turbine.

CONSTITUTION: A ring shaped frame 2 is provided so as to face a horizontal rotary shaft 1. Spokes 3 are linked between the frame 2 and the shaft 1. Pins 5 are protruded at both ends of the longitudinal direction of the water turbine blades 4 on the side of the shaft 1. Each pin 5 is pivotally attached to each spoke 3. At this time, each blade 4 is attached so that it is opened only to the upstream side with respect to the spoke 3 by a stopper and the like. Therefore, when the blade 4 is immersed into the water with the rotation of the water turbine, it is closed to the spoke 3 by a water flow V, and the turbine is rotated by the water flow. When the spoke 3 is moved to the water surface from the location directly below the shaft 1, the blade 4 is opened to the upstream side by the reacting force of the water, and the resistance of the water is escaped. Therefore the efficiency of the water turbine is improved.