PURPOSE: To prevent the damage of the slide part of a compressor by detecting concentration of a refrigerant flowing through a freezing cycle.

CONSTITUTION: A sensor part 12 wherein cylindrical light collecting lenses 17a and 17b are arranged to an optical axis support tool 16 in a manner to be positioned facing each other on one and the same line with a distance therebetween, and a measuring part 11 having a luminous module 13 and a light receiving module 14 are provided. Further, optical fibres 18 and 9 through which the luminous module 13 and the light receiving module 14 of the measuring part 11 are connected to the cylindrical light collecting lenses 17a and 17b, respectively, of the sensor part 12 are provided.