Abstract: A light travelling from a light transmission window (8) to a light receiving part (2) is transmitted first through a first light guide (11) of linear single core construction and next through a second light guide (12) which is bent and of multicore construction. Single core construction achieves a high light mixing effect, and accordingly the first light guide (11) makes a distribution of light intensity uniform. On the other hand, multicore construction has little light mixing effect, and raises little variation in distribution of incident light intensity. In other words, the second light guide (12) transmits the light to the light receiving part (2) without breaking the uniform distribution of light intensity achieved by the first light guide (11).