PURPOSE: To make it possible to execute calibrating work in a set state even when the sort of fluid to be used is changed by providing a sensor tube with a heating element, executing measurement in a different condition and finding out the characteristic constant of unknown gas.

CONSTITUTION: A CPU 17 controls the flow rate of gas flowing into a main gas passage 11 based upon a temperature difference between a pair of thermo- registers 16 arranged on the upstream side and downstream side of a sensor tube 12 for dividing gas to be measured. The sensor tube 12 is thermally changed forcedly by a heater 18 and gas with known characteristics is allowed to flow in accordance with the driven state or non-driven state of the heater 18. The 1st arithmetic means 17A finds out a device constant in a different condition based upon the gas flow rate measured in each condition and the temperature difference obtained by the thermo-registers 16. Then, unknown gas with a fixed flow rate is allowed to flow in the driven state and non-driven state of the heater 18. The 2nd arithmetic means 17B finds out the characteristic constant of the unknown 98S based upon the device constant under the different condition obtained by the means 17A.