A flange portion extending outward from a main shaft portion is disposed outside a side plate with respect to a position of an attachment member in a main direction when the attachment member is mounted to the side plate. A restriction slot is formed in the attachment member such that the flange portion can be fitted in and pass through the restriction slot. In the middle of the process where the attachment member is mounted to a second side plate in the main direction, the flange portion is fitted in the restriction slot, thereby the sheet detecting member is restricted from rotating, and one of a pair of projection portions of a sheet guide member is inserted in a hole of the attachment member.