In a planetary gear speed reduction mechanism includes a drive gear, a driven gear, a sun gear connected to the drive gear, an internal gear disposed around the sun gear, a planetary gear meshed with the sun gear and the internal gear, a bearing for supporting the planetary gear, a carrier pin for supporting the bearing and a planet carrier connected to the carrier pin for transmitting revolving motion of the planetary gear to the driven gear, the bearing has a cylindrical inner wall forming an annular grease reservoir around the carrier pin at opposite ends thereof. The bearing can be lubricated by the grease supplied from the simple grease reservoir for long time.