To provide a power generator having high efficiency, and requiring less construction cost and equipment cost, by directly generating electric power from wave or wind power.

In an impeller, blade surfaces are installed symmetrically, uniformly and radially on an outer shaft through which a core shaft and an inner shaft pass, (a) the length of the impeller substantially coincides with a wavelength of the wave, and (b) the tip of the wave substantially coincides with the tip of the impeller. The above operations are automatically conducted. (c) The whole or a part of a half surface is opened in a front half surface and a rear half surface on one of a pair of the blade surfaces. A valve function is given to the opening surfaces by a cover and a limited spring. The areas of the opened front and rear half surfaces are the same. The surfaces are opened in the same direction. Therefore, they are opened in an opposite direction when facing each other. The outer shaft can be rotated always in the same direction by axially symmetrical smooth force.