To restrain 0-degree light from being mixed into 1-degree diffracted light taken out by increasing the purity of spectral wavelengths at an acoustooptical element.

Source light enters the incident plane of an acoustooptical element 4 as a parallel beam by a light-source optical system including a mirror and a lens. 0-degree light 26, +1-degree diffracted light 22, and -1-degree diffracted light 24 from the acoustooptical element 4 are focused on the focal plane of the lens 20, and the +1-degree diffracted light 22 and the -1-degree diffracted light 24 enter respectively the ends 32, 34 of a branch optical fiber placed on the focal plane of the lens 20. Since the optical paths of the +1-degree diffracted light 22 and the -1-degree diffracted light 24 are spaced apart spatially, they are curved by prisms 28, 30, and the ends 32, 34 of the branch optical fiber are placed on the respective curved optical parts. The +1-degree diffracted light 22 and the -1-degree diffracted light 24 that enter the ends of the branch optical fiber are irradiated to a subject for measurement after being combined on the same optical axis at the other end of the branch optical fiber.