As previous processing of measurement in which gas to be measured containing,as gas components, carbon dioxide 13C02 and carbon dioxide 12C02, isintroduced into a cell, and in which the intensities of transmitted lightshaving wavelengths suitable for measurement of the respective gas components,are measured and then data-processed to measure the concentrations of the gascomponents, the air having a predetermined volume Va is sucked by a gasinjection device 21, a gas exhaust valve V6 of a cell 11 is closed and the airstored in the gas injection device 21 is transferred to the cell 11 filledwith the air at an atmospheric pressure, thereby to pressurize the cellinside. The pressure thus pressurized is measured as P. The cell volume Vc issubtracted from the product obtained by multiplying the sum. VO of the volumeVa and the cell volume Vc, by the ratio P0/P in which PO is the targetpressure of the gas to be measured at which a calibration curve has beenprepared for an isotope gas analysis and measurement, thus determining the one-time gas injection amount of the gas injection device 21. Thus, measuredconcentration variations based on changes in atmospheric pressure can becorrected.
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