PURPOSE:To cancel the difference in the quantity of light due to difference in the reflecting parts of a rotary flicker device in a signal processing system, by integer- multiple-integrating the detected signals of sample light and reference light respectively in response to the number of reflecting members of the rotary flicker device. CONSTITUTION:A monochromatic light from a slit 3 of a spectroscope is reflected by the reflecting part of a rotary flicker device 10 to become a reference light and, simultaneously, become a sample light by transmitting through the sample provided in the transmission part, and the both lights are alternately detected by a detector 6 through a similar flicker device 10' which turns in synchronism with the flicker device 10. The both detection outputs of this detector 6 are passed through an A/D converter 30 and a memory 31 to be integrated by the integer multiples in a digital operation unit, depending on the number of the reflecting parts in the flicker devices 10, 10' thereby becoming a detection output with the difference in the quantity of light due to difference of reflecting parts canceled each other. Therefore, spectral analysis is possible without being influenced by the difference of the reflecting parts in the flicker devices.