PURPOSE:To make it possible to simultaneously measure two wavelengths which are close to each other, by dividing an auxiliary Rowland circle into a top section and a bottom section. CONSTITUTION:A light having entered from an entry slit 1 is diffracted and made to form an image by a concaved surface diffraction grating 2, and a spectrum line forms an image on a Rowland circle 6. And the Rowland circle 6 is divided into top and bottom sections by an image-dividing mirror 3, and the spectrum line is also divided into top and bottom sections to form images on Rowland circles 7 and 8. These Rowland circles 7 and 8 are provided with exit slits 4 and 5, respectively, so that if, for instance, 4, 5 or 2 is scanned along top of the Rowland circles, variously different wavelengths can be measured. Further, since the auxiliary Rowland circles are divided into top and bottom sections, it is physically possible to prevent the exit slit from being hit by two wavelengths which are close to each other.