To provide a low-noise circuit which effectively removes 1/f noise of an amplifier.

A detection system includes an infrared detector 12 provided to a nondispersive infrared detection system 10, a modulator 21 which chops the DC signal from the infrared detector into a component having a 1st specific frequency, an amplifier 24 which amplifies the signal from the modulator, a demodulator 26 which is coupled with the output of the amplifier and synchronized with the modulator to provide a DC level output signal, and a filter which is coupled with the output of the demodulator and removes a signal component exceeding a 2nd specific frequency, and an input signal is modulated with a frequency of about 1 kHz and then the signal is demodulated and filtered to provide an analog DC output level. Thus, 1/f noise of the amplifier 24 can effectively be removed by selecting the modulation frequency higher than a frequency where the level of the 1/f noise is a problem.