PURPOSE: To enable the measurement of the concentration of carbon dioxide in the exhaust gas during fermentation process by housing a diaphragm type gas electrode into a cylinder having a heat-pressure-resistant gas permeating membrane for permeating a gas to be inspected.

CONSTITUTION: The opening of a cylinder 1 made of a heat and pressure resistant material such as stainless steel pipe is closed with a membrane large in the permeability of gas to be inspected such as a poros polytetrafluoroethylene resin membrane and a silicone ruber film and a heat-pressure-resistant gas permeating membrane 2 for permeate the gas to be inspected formed by alternately laminating unwoven cloth of cynthetic fiber. Numeral 4 indicates a diaphragm type gas electrode which is allowed to be retained in a cylinder 1 by tightening a box nut 11 onto the cylinder 11 surrounding a cap 10 from thereoutside 10 in such a manner that a diaphragm 5 provided at one end thereof gets close to the inner surface of the gas permeating membrane 2. To use this apparatus, the cylinder 1 is inserted into a hole 15 provided on the wall 14 of a fermentation tank or a gas exhaust tube half way so that the gas permeating membrane 2 is located therein 1 and a flange 17 or the like provided at the center of the outer surface thereof 1 is mounted on the wall 14 with a box nut 16 or the like.