PURPOSE: To obtain gaseous CO of a desired concn. for quantitative determination by mounting a mixture of a sulfate with activated carbon on a substrate in a case having an intake and a blowoff hole, covering the mixture with a gaseous CO permeable cover, and controlling electric power supplied to a heater for heating in the mixture with a circuit for heating.

CONSTITUTION: A mixture 5 of a sulfate such as BaSO4 with activated carbon is mounted on a substrate 4 in the case 1 of a gaseous CO generator having an intake 2 and a blowoff hole 3, and the mixture 5 is covered with a gaseous CO permeable cover 6. A heater 7 for heating is embedded in the mixture 5, and by supplying electric power to the heater 7 under control with a circuit 8 for heating through leads 9, the mixture 5 is heated to generate gaseous CO. The generated gaseous CO enters the case 1 through the cover 6, it is diluted to a constant concn. with air fed by a blower 10, and the diluted gaseous CO blows off from the hole 3.