To provide a gas concentration measuring device capable of detecting, when moisture in a liquid state has invaded into an apparatus or a large quantity of moisture in a gaseous state has invaded into the apparatus, the invasion by using existing constituents.

A gas concentration measuring device comprises: a light source; a sample cell that is arranged on a light path from the light source and into which sample gas is introduced; an interfering component detector that receives light from the light source transmitted by the sample cell and detects the intensity of light in a wavelength band matched with the absorption spectrum of the interfering component; a main detector that receives light from the light source transmitted by the sample cell and detects the intensity of light in a wavelength band matched with the absorption spectrum of the component to be measured; an arithmetic operation unit that calculates the concentration of the component to be measured on the basis of the intensity detected by the main detector and the intensity detected by the interfering component detector; an invasion detector that detects the possibility of invasion by any performance degrading substance on the basis of the intensity detected by the interfering component detector; and an invasion preventer that, when the possibility of invasion by any performance degrading substance has been detected, blocks further invasion of the performance degrading substance.