symbols b and d refer to the points at which signals were detected during expiration phases subsequent to points a and c respectively said signals having been reduced in accordance with the concentration of co 2 in the exhaled gas consider for example point a according to the invention a maximum value va is detected at point a during the inspiration phase and stored then an output value vb is detected at point b during the subsequent expiration phase and subtracted from va this procedure is repeated for each cycle of inspiration and expiration phases this embodiment in which a maximum value for each inspiration phase is stored and held such that the difference from an output value for the subsequent expiration phase is determined is effective in the case where the drift to be compensated is mild or small besides the maximum value in each inspiration phase is not limited by the maximum value of present inspiration phase namely for example when a value which corresponds to the maximum value is defined on a line connected between the adjacent maximum values of inspiration phase by a successive line or curve the measurement value is not changed in a step manner in the case where if the drift.