Abstract: A wind power prime mover in which a windmill assembly (1) comprising semicylindrical wind receiving members (16) vertically erected on a plurality of equally spaced points on the circumference centering on a vertical rotary shaft (3) in such manner that semicylindrical open parts of said wind receiving members are directed to the same circumferential directions with respect to individual tangents to said circumference is of knock-down assembly construction with the major parts thereof joined to or disjoined from each other with bolts; and the top and bottom of said rotary shaft (3) are supported by bearings (10), (11) in a supporting framework composed of a support pillar (2), central support frame (8), upper frame (6), and lower frame (9), whereby a wind power prime mover hardly affected by instantaneous variation of wind velocity, comparatively free in selection of installation site, and manufacture, transport, assembly, and maintenance can be provided easily.