A first optical system (10) according to the present disclosure includes a first lens (111) that guides light (LO) to a diffraction grating (3), a second lens (112) that collimates first diffracted light (L1) that was focused at a first focal point (f1), a pair of first mirrors (12, 13), a third lens (113) that focuses the first diffracted light (L1) at a second focal point (f2), and a fourth lens (114) that guides the first diffracted light (L1) that was focused by the third lens (113) to the diffraction grating (3). The first lens (111) and the fourth lens (114) have a substantially identical first focal length. The second lens (112) and the third lens (113) have a substantially identical second focal length. A first distance along an optical path from the first focal point (f1) to the second focal point (f2) is determined by a first predetermined condition.