An image writing unit causes, upon being set in a write position, an optical system unit to be movable and positions the optical system unit, with a hitting portion of the optical system unit hitting a blocking portion in a chassis. A holding unit includes, in a state in which the image writing unit is set in the write position, a portion defining an opening that faces either one of an air intake unit and an air exhaust unit. The air intake unit blows an intake air to the optical system unit through the opening, or the air exhaust unit draws an exhaust air around the optical system unit through the opening.