The image forming apparatus includes an image forming unit installed in a main body of the image forming apparatus and a tray stack including stacked media supplying units for supplying the media piece from selected one of the stacked media supplying units to the image forming unit. The tray stack is detachable from and attachable to the main body. The tray stack includes a register installed only in uppermost one of the stacked media supplying units. The register functions to correct skew of the media piece with respect to a transport direction along which the media piece is transported and then feed the media piece to the image forming unit.