To enhance durability of a gear transmission mechanism by eliminating play in a gear engagement due to the backlash between the gears (gear clearance) of the gear transmission mechanism in a variable pitch mechanism of blades or a nacelle turning mechanism and further to reduce damages to a gear tooth surface caused by slide of the tooth surface or impact thereon due to slight vibration, the gear transmission mechanism is constructed such that a first pinion gear 20a and a second pinion gear 20b are provided to mesh with a gear wheel 18 respectively and rotated by a first drive motor 32a and a second drive motor 32b respectively, and a controller 36 controls so as to eliminate play in the engagement of the internal gear and the pinion gear due to backlash between the internal gear 18 and the first pinion gear 20a, thereby avoiding generation of a slide and impact force on the gear transmission mechanism even when the external force or the wind acts on the blade 16.