An transfer belt device includes a roller support supporting transfer rollers, a cam driving shaft having a cam, the cam having capability of swinging the roller support to move the transfer rollers toward and apart from color photosensitive drums to thereby switch a transfer belt and the color photosensitive drums between a state where the transfer belt is in contact with the color photosensitive drums and a state where the transfer belt is out of contact with the color photosensitive drums, a tension roller kept in contact with a surface of the transfer belt to exert tension on the transfer belt, and a pivotal axis on which the tension roller is swingably supported, the pivotal axis being common with the cam driving shaft.