PURPOSE: To provide a gear both having a built-in mechanical fuse device properly operating over a long term, and incorporating an improvement to eliminate the need of building a mechanical fuse device into a joint section with other members.

CONSTITUTION: This gear is equipped with an internal gear member 26 having an indentation 36, and an outer gear member 28 coupled to the member 26, faced to the external surface 32 thereof along an internal surface 34 and having a keyway 40. Also, a plurality of teeth 30 are formed on the external surface of the member 28. Furthermore, a ceramic key 42 is inserted in the indentation 36 and the keyway 40 for connecting the members 26 and 28 to each other for torque transmission. The key 42 is designed to break, when a load transmitted between the two members 26 and 28 exceeds the preset value, thereby disconnecting the members 26 and 28 from each other.