In a fixing device, a heat roller and a fixing gear for rotating the heat roller are arranged concentrically, and coupled to each other by inserting a key formed in the fixing gear into a keyway formed in the heat roller. The keyway is designed such that the key is allowed to move along a direction in which the heat roller is rotated. For example, the keyway is formed by notching a free end of a one-side rotary shaft of the heat roller so as to extend along the direction in which the heat roller is rotated.