PURPOSE: To improve the contact of gas with a catalyst and also, to improve reliability, by constituting so that gas is sent to a powdered and granular oxidizing catalyst after diluting at a constant ratio by a combustion supporting gaseous body and a temperature by oxidizing reation is detected by a temperature sensor.

CONSTITUTION: A diaphragm 3 is provided in the inner part of cases 2a, 2b of a constant rate dilution apparatus 1 and the first airtight room 4a and the second airtight room 4b are formed and then, communicating holes 10a, 10b having an inner diameter corresponding to a dilution ratio of a gas G by a combustion supporting gaseous body S, are provided between each room 4a, 4b and an adjacent mixing room 9. The gas G from a gas main pipe 8 and the body S sent from the outside are supplied to each valve room 7a, 7b and the diaphragm 3 is moved in accordance with the difference of pressure between both rooms 7a, 7b. Then, the gas G and the body S are supplied to the room 9 at a constant ratio and the dilution at a constant rate is always carried out. Mixed gaseous body M is supplied to a calorimeter for gas 12 and oxidizing reaction of the gas M by an oxidizing catalyst is carried out and then, oxidized gas is discharged as a waste gas RG.