A light spectrum detecting apparatus according to the present invention comprises a photodetector (13) and a light transmitting plate (13b) covering a light receiving surface (13a) of the photodetector (13), and in the apparatus, a front surface S of the light transmitting plate 13b is inclined and, among incidents lights, any light once reflected by a back surface T of the light transmitting plate (13b) travels thereafter, with being totally reflected by the front surface S and the back surface T of the light transmitting plate (13b), to a side surface 13d of the light transmitting plate (13b). Any stray light can be prevented from entering the photodetector (13).