A gear device has an outer cylinder fixed to a first mating member and a crankshaft inside the outer cylinder. An external gear is mounted to an eccentric portion of the crankshaft and meshes with internal teeth of the outer cylinder. A carrier is fixed to a second mating member. Rocking rotation of the external gear rotates the carrier coaxially to the outer cylinder. A cylindrical body is inserted in an axial through-hole of the carrier. The carrier has a first portion on one end of the cylindrical body and a second portion on an axially intermediate portion of the cylindrical body. The other end of the cylindrical body extends toward a corresponding mating member while protruding axially beyond the second portion. An outer diameter of remaining portions of the cylindrical body do not exceed an outermost diameter of the portion on which the first and second portions are fit.
