An EGR ratio measuring device comprises a pair of nondispersive infrared gas analyzers that correct a water influence and measure a concentration of CO2 in a gas containing water, an intake air introduction line that is connected to an intake air pipe of an internal combustion engine and introduces a part of the intake air into one nondispersive infrared gas analyzer without removing the water, an exhaust gas introduction line that is connected to an exhaust gas pipe of the internal combustion engine and introduces a part of the exhaust gas into the other nondispersive infrared gas analyzer without removing the water, and a temperature adjusting mechanism that keeps a temperature of whole of the introduction lines and a temperature of the nondispersive infrared gas analyzers so as not to condense dew.