To provide a simple method for measuring the mass flow of a gas, a using method of its device, and a method for sufficiently avoiding the pollution of the device being used.

A first heating element is heated to a temperature T1 selected higher than the temperature Ts of a gas flow, and a second heating element is likewise heated to a temperature T2 selected higher than the temperature Ts of the gas flow and different from T1. The temperatures T1 and T2 of both heating elements are kept constant respectively, and the cooling of both heating elements by the gas flow is avoided while the heat output P1 required to keep the temperature T1 constant and the heat output P2 required to keep the temperature T2 constant are changed, and the heat output P1 of the first heating element and the heat output P2 of the second heating element are detected. The difference between the heat outputs P1 and P2 is obtained, and this difference is evaluated as a magnitude for the mass flow of the gas by this measuring method.