To obtain a Golay cell in which the time-dependent stability of a sensitivity characteristic can be enhanced by a method wherein the least a gas sealing chamber is installed inside an airtight case in which a gas of the same kind as a gas inside the gas sealing chamber is sealed at a nearly identical concentration and the leak to the outside of the gas inside the gas sealing chamber is reduced.

A Golay cell 116 is installed inside an airtight case 140. 13CO2 as a gas of the same kind as a gas inside a gas sealing chamber 124 is sealed, at a nearly identical concentration, inside the airtight case 140. A pressure inside the gas sealing chamber 124 at the Golay cell 116 and a pressure around it can be kept at a nearly identical pressure. As a result, even when the 13CO2 as the gas inside the airtight case 140 is leaked a little to the outside, the differential pressure between the gas sealing chamber 124 and the airtight case 140 around it can be reduced, and the leak, to the outside of the gas inside the gas sealing chamber 124, generated due to the differential pressure of a gas concentration can be reduced. Since the leak of the gas is reduced, the time- dependent stability of the sensitivity characteristic of the Golay cell 116 can be enhanced.