To enable an installation on a roof by providing support frames extending symmetrically right and left around a rotational axis along a wind receiving face, on upper and lower ends of a Savonius type windmill, and providing propeller type floating force utilizing vanes on both ends of the support frames vertically in parallel with the rotational axis.

Support frames 2, 2 are provided on upper and lower end rotational faces of a Savonius type windmill vanes so as to extend symmetrically right and left around a rotational axis 4 along a wind receiving face. On both ends of the frames 2, 2, propeller type vanes 3, 3 are mounted vertically in parallel with the rotational axis 4. In this windmill, the vanes 1, 1 receive resistance at first and start at low rotational speed. Along with the start of rotation, a peripheral speed ratio increases on account of a difference between rotation radii of the vanes 3, 3 and it becomes suitable for the high efficiency conditions of Darius type vanes. Accordingly, this windmill implements both low peripheral speed operation of the Saponius type and high speed peripheral operation of the Darius type. Also with a relatively flat structure for horizontal rotation, it can be mounted on a roof easily.