An optical unit includes a frame; an optical member having a rectangular plate-shape; and a pair of mount portions formed in the frame, each holding a side of the optical member. Each of the mount portions includes an insertion slot into which an end of the optical member in the width direction is inserted. The insertion slot is formed between first and second walls of the frame. The first wall includes a thickness direction positioning surface that is in contact with one of the surfaces of the optical member in a thickness direction. The second wall includes an inclined surface. The optical member is fixed to the frame using an adhesive in a state in which the one of the surfaces of the optical member is pressed against the thickness direction positioning surface and an end of the optical member in the height direction is pressed against the inclined surface.