PURPOSE:To reduce the backlash of a planetary gear mechanism and improve the durability by forming a rugged face on the mutual contacting face with the gears in a planetary reduction gear and forming a thinner film than the height of the ruggedness onto the rugged face through formation processing. CONSTITUTION:The contacting face of an external tooth gear 5 or an external gear is ground in the axial direction and rugged faces A, B are formed in the direction perpendicular to the grinding direction. The rugged faces A, B are formed with a thinner phosphate film C than the height of the ruggedness through formation processing. Consequently, the lubricant is held on the rugged face while abrasion is reduced resulting in reduction of the backlash of the planetary gear mechanism and improvement of the durability.
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