PURPOSE: To increase the output efficiency of a Saponius type windmill by pivotally supporting the both edges of the vane piece of a Saponius type windmill and opening the gap between the vane pieces when the vane of the windmill is back-opposed to fluid.

CONSTITUTION: The vane piece 5 of a Saponius type windmill has a long shape, and pivotally supported by the pivotal shafts 6 installed at the both edges, and performs the pendulum movement around the shaft. The vane piece 5 has a short piece 5a on one side of the axis line of the pivotal shaft 6 and has a long piece 5b on the other side, and the gap between the contiguous vane pieces can be opened and closed by the pendulum movement in one direction of the contiguous vane pieces. Therefore, when the vane of the windmill is back-opposed to the fluid, the gap between the vane pieces is opened, and fluid smoothly passes between the vane pieces, and the output efficiency of the windmill can be improved.