PURPOSE: To reduce the temperature change of a photoelectric conversion element to improve the measuring accuracy of the device, by providing a heat insulating member covering the photoelectric conversion element or a receptacle receiving it.

CONSTITUTION: A receptacle 15 with a built-in photoelectric conversion element is covered, except the fitting port 4 of an optical fiber, with heat insulating parts 2a, 2b, and the photoelectric conversion element is insulated from ambient temperature fluctuations, the convection current of air, etc. By this, the temperature of a photoelectric conversion element module 1 with the built-in photoelectric conversion element is stabilized, and the detection accuracy of Raman scattering light can be improved.