To provide a gas concentration measuring apparatus enhanced in measuring precision.

The gas concentration measuring apparatus is an infrared absorbing type gas concentration measuring apparatus, which is constituted so that the infrared rays emitted from an infrared ray source are intermittently thrown on a sample cell, into which a sample gas is introduced, through a rotatable chopper and a gas correlation cell and the quantity of the infrared rays transmitted through the sample cell is detected by a detector to operate the concentration of the measuring target gas in the sample gas. In this gas concentration measuring apparatus, the data of a predetermined period of the output signal of the detector is taken in an arithmetic means using the change timing of the rotary position detecting signal of the chopper (rotary sector) to calculate the concentration of the measuring target gas. A photosensor P for detecting the rotary position of the chopper 12 is arranged on the straight line, which connects the rotary axis C of the chopper 12 and the optical axis R of the infrared ray source at the position in the vicinity of the optical axis R.

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