The present invention realizes (i) a gas flow measuring device which can measure a gas flow of a low-boiling gas in a mixed gas including two types of gasses having different boiling points, and (ii) a method of measuring a gas flow easily but highly precisely, using the gas flow measuring device. The gas flow measuring device includes a condensing tank for condensing a mixed gas in which two types of gasses having different boiling points are mixed with each other, and a flow meter for measuring the flow of the mixed gas. After condensing the mixed gas in the condensing tank so as to cause the gas flow of the high-boiling gas to be known, the flow is measured using the flow meter so that the gas flow of the low-boiling gas is worked out.