A fixing unit fixes a plate-shaped member to a fixing base member. The fixing unit includes: a pressing unit configured to press the plate-shaped member toward the fixing base member; and a plurality of positioning units, installed at the fixing base member to be in contact with side surfaces of the plate-shaped member, and configured to place the plate-shaped member with respect to the fixing base member. Each of the positioning units includes: a shaft to be installed at the fixing base member; and a slide part movable along the shaft, and the slide part includes a contact part to be in contact with one of the side surfaces of the plate-shaped member and a clearance part formed on the contact part to have a smaller width than that of the contact part.