Abstract: In a Littrow spectroscope including a concave mirror for reflecting light from a source, a diffraction grating for further reflecting the light reflected by the mirror, the grating being made rotatable to allow the mirror to re-reflect the light therefrom, and an exit on which the light re-reflected by the mirror is focused; a stray-light suppressor in the form of a strip is disposed in front of, and horizontally in parallel with the longitudinal central axis of, the mirror. The suppressor is held in front of the mirror at a distance of (L1-L2), where L1 is the distance between the mirror and the grating and L2 is the distance between the grating and the exit. The suppressor portion upon which the stray light from the diffraction grating impinges is tapered.