PURPOSE: To detect carbon dioxide gas sensitively by using a carbonic material as a detecting material.

CONSTITUTION: A carbon-dioxide detecting element 1 is formed as follows: a carbon-dioxide detecting material 3 is applied around a conductive material 2 comprising a platinum coil as a cover; a voltage is applied across terminals 4 and 5 which are extending from both end parts of the conductive material 2; and a current is conducted through the conductive material 3. Said carbon- dioxide gas detecting element and resistors R1 - R3 are assembled together, and a Wheatstone bridge circuit is formed. The specified voltage is applied from a power source E, and the current is conducted. The compensating resistor R3 is adjusted so that the current value of a detector D becomes 0. The change is resistance of the detecting element 1 is measured. Thus the detection of the carbon dioxide and the measurement of the concentration are performed.