To make a coming-off prevention structure of a transmission mechanism compact by forming a coming-off prevention means on a side face in a protection cover which covers the transmission mechanism in which a transmission member is annularly provided in the vicinity of a shaft end.

A shaft 1 protrudes on the outside from a side plate 4 of a device, a transmission member 2 such as a gear is annularly provided in a protrusion part, and a protection cover 3 is covered from the outside of a shaft end 1a. A protrusion 3a as a coming-off prevention means which prevents the transmission member 2 from coming off to the outside from the shaft 1 is formed on an inner face of the protection cover 3. The protrusion 3a, is formed into a circumferential shape to prevent the interference with the shaft 1, and the protection cover 3 is integrally formed by resin. Consequently, a distance D' from an outer face of the transmission member 2 to an inner face of the protection cover 3 is shortened, a length in the transverse direction of the protection cover 3 can be reduced, and a coming-off prevention structure of a transmission mechanism can be made compact.