PURPOSE: To enable using a cylindrical sample having a constant circular section, and prevent all lights incident on a cell wall from entering a photodetector.

CONSTITUTION: The light from a light source 10 passes an aperture 16, and is captured and dispersed by a holographic grating 14, and converged on a surface passing an entrance aperture 22. The light source 10 has a narrow radiation band, functions as an entrance slit of a monochromator, and lights having various wavelengths are converged on places 19a-19c. The focal length of a cell entrance lens 26 is so selected that the image of a mask part member 16 is formed on a place 42 in the vicinity of an exit of a cell hole 23. The envelope 40 of a ray intersecting the hole 23 forms a convergent taper. The ray leaves the cell through a window 30 which forms a beam 50 entering perfectly the sensitive band of a photodetector 60. All rays selected from the light source radiation by an aperture stop 16 and a field stop 22 reach the detector without colliding against the cell wall, independent of refractive index.