An exposing unit includes a holding member, an abutting pin protruding on both sides in the vertical direction at the rear side of the holding member, a second support portion provided further to the rear side than the holding member, and a movement mechanism that causes the holding member to move between an exposure position and a retracted position. The second support portion has a gap formed where the rear side of the holding member moves in the vertical direction, and this gap restricts movement of the holding member in the left-and-right direction. The holding member that is moved from the retracted position toward the exposure position by the movement mechanism is guided by the gap formed in the second support portion so that the abutting pin abuts a fitting portion.