A gear mechanism is arranged in such a manner that, a driven gear, which is detachable with respect to a first rotating shaft around which the driven gear rotates, is moved in a fitting direction along the axis of the first rotating shaft, so as to be fit with the first rotating shaft, and consequently the driven gear engages with a driving gear that has been positioned. In this gear mechanism, when the driven gear is moved in the fitting direction and the driven gear and the driving gear are about to engage with each other, the driven gear and the first rotating shaft are arranged so as to separate from each other in a direction perpendicular to the fitting direction.