PURPOSE: To obtain a device which has the scattered light converging mechanism of a simple constitution although a sample moves linearly by making scattered light into parallel transmitted light by a cylindrical lens.

CONSTITUTION: Part of scattered light 7 projected from a beam spot 7, i.e., almost all of light passed through a cylindrical lens 22 become parallel to the optical axis plane of the lens 22. An end surface 26a held by the holder 2 of an optical fiber bundle 26 is so formed as to receive the light 27 efficiently, and the other end of the fiber bundle 26 is connected to a photoelectric converter 28. The end surface 26a is arranged in parallel to the plane part 23 of the lens 22. The converter 28 converts the quantity of the transmitted light 27 incident through the fiber bundle 26 into an electric signal 28a, which is outputted. Then, the reflected light of the illumination light 6 from the sample surface 3a is never incident on the fiber bundle 26. Consequently, the S/N of the signal 28a is improved.