PROBLEM TO BE SOLVED: To provide a method for measuring a CFgas concentration and a device for measuring a CFgas concentration capable of easily detecting, with high reliability, the CFgas concentration of a gas to be inspected containing the CFgas and a CFgas.SOLUTION: The concentration of a CFgas is measured by correcting gas detection output corresponding to the luminous energy of an infrared ray in a first wavelength region having a center wavelength of 7850±50 nm, the gas detection output being obtained by infrared ray detection means with respect to a gas to be inspected containing the CFgas and a CFgas. The correction is made by correction output obtained by (1) a procedure for acquiring the concentration of the CFgas on the basis of the absorbance of an infrared ray in a second wavelength region having a center wavelength of 9035±50 nm, and (2) a procedure for acquiring, as the correction output, the gas detection output obtained by checking the obtained concentration of the CFgas against a calibrat
