A detector for a single beam pneumatic infrared analyzer having two light receivers, a front receiver and a rear receiver positioned in series in the path of infrared light rays coming from a sample cell and each of which has a gas enclosed therein and used for measuring the concentration of a sample flowing through the sample cell by comparing the different absorption spectra of the infrared rays hitting the two receivers, and a screening device in front of the front light receiver for screening out the infrared rays in only the peripheral portions of the light, along the light path entering the front receiver.