A method and apparatus for applying infrared radiation to a respiratory gas and detects a signal associated with the quantity of transmitted radiation for measuring the concentration of CO2 in the respiratory gas. The capnometer detects a maximum value of the detection signal from the thermal detector during the present inspiration phase, stores the detected maximum value in RAM, performs the first approximation, Kalman filtering or some other suitable method to compute a corrected value that is valid for the time being until a maximum value is detected in the next inspiration phase, and determines a density signal by calculating the difference between the corrected value and the detection signal.