A reflecting mirror is disposed behind a single Fabry-Perot interferometer non-perpendicularly relative to the optical axis of the interferometer. The light transmitted through the interferometer parallelly to the optical axis is reflected by the reflecting mirror and, therefore, caused to pass reversely through the interferometer at an angle relative to the optical axis. In this way, a single interferometer offers two different mirror gaps, one for the light path in the forward transmission and the other for the light path in the reverse transmission through the interferometer. The forward and reverse transmission of the light through the interferometer at different mirror gaps serves to enlarge the free spectral range to a notable extent.