Planetary gear reduction mechanism with tapered tooth end

A planetary gear speed reduction mechanism (100) has a sun gear (1), an internal gear (2) and a plurality of planetary gears (3) disposed between both gears (1, 2). Each tooth (10) of the sun gear (1) has a tapered portion (11) formed at a front end thereof such that the tapered portion (11) tapers off in the axial direction. The tapered portion (11) has a pair of symmetrical inclined surfaces (11b) whose width is tapered off in the circumferential direction from a tooth surface (10b) of each tooth (10) and a tapered surface (11a) whose peripheral surface is tapered off in the radial direction from an end of a tooth crest surface (10a). In the process of inserting the sun gear (1) into a space among the planetary gears (3), the sun gear (1) is inserted thereinto under the guide of the inclined surface (11b) by slightly rotating the sun gear (1).