A transfer device is configured to include a drive roller, a driven roller and a transfer belt supported between these rollers and provided with first and second guide ribs extending circumferentially at both side edges on the undersurface thereof and a drive roller, so as to prevent the transfer belt from skewing by making the drive roller and driven roller abut at both ends against first and second inner circumferential surfaces of the first and second guide ribs. The drive roller and the driven roller are specified to have a length approximately equal to each other and shorter than the spacing between the first and second inner circumferential surfaces and arranged such that the first spacing between a first end of the drive roller on the drive gear side and the first guide rib is wider than the second spacing between a second end of the driven roller on the drive gear side and the first guide rib.