PURPOSE: To obtain a planetary reduction starter easy to assemble where the lubricant will never flow to the outside by covering the outside sun gear in a planetary gear system with resilient rubber member continuous to the left side of the opening, the outercircumference and the right side of the opening of the gear.

CONSTITUTION: A molded rubber gear cover 26 covering the left sideface 6a of the opening, the outercircumference 6b and the right sideface 6c of the opening of an internal tooth ring gear 6 is held by the center brackets 14, 18 and pressure contacted with predetermined resiliency against the center bracket. Consequently, the lubricant sealed in the planetary gear system will never flow out to exhibit stable function for long term. While since the teeth of the internal tooth ring gear 6 are opened in the axial direction, assembling of the planetary gear 7 is facilitated.