PURPOSE: To prevent a snap ring from falling down and to facilitate its disassembly by providing a rotary part which has inserted through a shaft and rotates in a body with a rotation checking projection entering between both the ends of the opening part of a E-type snap ring fitted in a groove on the shaft.

CONSTITUTION: A E-type snap ring 3 is fitted in a ring-shaped groove 17 formed at the end of a shaft 10 inserted through a large gear 14, and the end of the large gear 14 is protrusively provided with a projection 14a so as to enter the opening of the snap ring 3. Thus the E-type snap ring 3 does not rotate relatively to the shaft 10 and the large gear 14 and fall down, and its dismounting work for disassembling and repairing can be easily carried out.