A gear mechanism includes a first gear, a second gear, an idler gear and a mover/stopper. The idler gear has an inner circumference with a centrally located inner space and an outer circumference rotatable relative to the inner circumference. The mover/stopper includes a bolt inserted through the inner space in a loosely fitted state and having a shaft part capable of assuming different radial positions relative to the inner space, and a receiving part holding the idler gear via the bolt. The idler gear and the bolt are configured such that the idler gear is allowed to be radially displaced relative to the bolt and to be tightened to the receiving part at a meshing engagement position by the bolt to be radially fixed and such that the inner circumference is allowed to be axially held between the receiving part and the bolt to be fixed.