PURPOSE: To measure the concentration of carbon dioxide gas accurately all the time by using detected vapor pressure, detected temperature and vapor- pressure and temperature correcting parameters, and operating the concentration of the carbon dioxide.

CONSTITUTION: A temperature detector 2 for detecting the temperature of a liquid sample is provided at a pipe 1. A sampling pipe 3 is branched from the pipe 1 and communicated to a nozzle pressure sensor 5 for measuring vapor pressure through a booster pump 4. The vapor pressure of the liquid sample is detected with the sensor 5. The detected vapor pressure is transduced into a current value in a P/I (pressure/current) transducer 6. The value is inputted into an operating part 7. The detected temperature of the liquid sample is also inputted from the detector 2 in the operating part 7. In the operating part 7, the concentration of the carbon dioxide which is dissolved in the liquid sample is finally operated based on the detected vapor pressure from the transducer 6 and the detected temperature from the detector 2. The measured concentration of the carbon dioxide gas is indicated on an indicator 8 and fed back to the manufacturing line for process control.