A gear mechanism comprises a first gear which has a portion with no teeth and rotates, a second gear which engages the first gear and a cam mechanism which includes a cam portion provided on one gear of the first gear and the second gear and a cam contact portion provided on the other gear of the first gear and the second gear and making contact with the cam portion. When the second gear moves from a position in which the second gear engages the tooth portion of the first gear to a position in which the second gear opposes the portion with no teeth of the first gear, the cam mechanism acts to bring the axes of the first gear and the second gear away from each other.