PURPOSE: To restrain the influence of harmful gas to prevent the drop of accu racy and responsiveness of a sensor, and to measure stably in a long term, by providing a filter made of zeolite, at an interval from a carbon dioxide sensor element and by treating the gas contacting the sensor element.

CONSTITUTION: In an opening part 5 for gas intake of a housing 4 or between the opening part 5 and carbon dioxide sensor element 2, a filter 3 made of zeolite is provided at an interval to the element 2. On this occasion, it is preferable that the filter 3 is provided at the interval of 0.5-30mm to the element 2 so as not to be heated not less than about 150°C by a heating means with which the element 2 is provided. By this, the excessive heating of the filter 3 is prevented, organic gas is converted into carbon dioxide by the filter 3, a bad influence to measurement is eliminated, an error of a measured value is reduced, and the performance stable for a long term can be displayed.