A fuser device for fusing a developer image on a medium by applying heat includes an endless fuser belt, a belt supporting part that has a contact face shaped in an arc centering on a first central axis and contacts with the inner circumferential face of the fuser belt on the contact face, a driven ring that is disposed on at least one side of the fuser belt in the width direction of the fuser belt, and a ring supporting part that has a contact face shaped in an arc centering on a second central axis and contacts with an inner circumferential face of the driven ring on the contact face, wherein the first central axis is shifted from the second central axis.