To provide an infrared gas analyzer capable of analyzing in a short time, reducing a cost, and saving a space.

Two cells having different lengths, one of which is a long cell 2 and the other of which is a short cell 3 are used as sample cells and connected in series, and sample gas is introduced therein. Each cell is arranged on separate optical paths, and since an infrared ray is introduced alternately into the two cells 2, 3 by chopping of infrared light caused by rotation of a shielding plate 9, a detector 6 can output by time sharing signals from each sample cell 2, 3, and measurement by the two cells having different lengths can be executed simultaneously by using a single light source 1 and the detector 6, and the signals from the two cell 2, 3 can be obtained simultaneously and independently. A signal obtained from the sample cell having the length fit for the sample gas concentration is separated from among the signals from the detector 6, to calculate the concentration of the sample gas. Hereby, switching of a passage to a cell having the different length, use of plural infrared measuring parts, preliminary concentration measurement or the like become unnecessary relative to the sample gas having any concentration.