An optical system includes a light guide member having a light-incident surface, a first surface, a light-emitting surface facing the first surface, a first end surface, a second end surface, a plurality of collimating lenses disposed on the light-incident surface and being arranged along a first direction, and a plurality of prism pieces. The plurality of prism pieces include two or more prism pieces having different inclination angles with respect to the light-incident surface depending on positions of the prism pieces in at least the first direction, and each of the inclination angles of the two or more prism pieces is an angle formed between the first direction and the longitudinal axis of a corresponding prism piece. Each of the inclination angles of the two or more prism pieces increases as the corresponding prism piece is positioned closer to any of the first end surface or the second end surface.