end portion of the side wall 61 includes a sliding face sliding portion 63 which substantially extends in the horizontal direction and has contact with a lower face of a rib locking member 64 provided in the main body of the image forming device since the sliding face 63 is formed in the upper end portion of the side wall 61 the sliding face 63 and the platen 53 move together the height of the sliding face 63 is set such that the sliding face does not have contact with the rib 64 when the platen 53 is locked in a lower position as illustrated in fig4 the sliding face 63 includes a circular stepped portion 63 a which has contact with the rib 64 provided in the main body of the image forming device when the platen 53 is inserted into the main body of the image forming device without being locked in a lower position the height of the sliding face 63 is also set such that the platen 53 is pushed down to the locking position by the rib 64 after passing through the stepped portion 63 a when the platen 53 is set in the main body of.