An EGR ratio measuring device to measure an EGR ratio of an internal combustion engine based on a concentration of CO 2 in an intake air introduced into a combustion chamber of the internal combustion engine and a concentration of CO 2 in an exhaust gas discharged from the combustion chamber, and comprising; a pair of nondispersive infrared gas analyzers (11, 12) that have a function of correcting a water influence and that are adapted to measure a concentration of CO 2 in a gas containing water, an intake air introduction line having a fluid equipment (21, 31, 41 51,61, 71) including an oil removing filter, a regulator, a suction pump, a filter, and a flow meter in this order from the upstream, wherein the intake air introduction line is connected to an intake air pipe at a sample point at a downstream of a Junction of a circulation path of the internal combustion engine and that Introduces a part of the intake air into one of the nondispersive infrared gas analyzers without removing the water, an exhaust gas introduction line having a fluid equipment (22, 32, 42, 52, 62, 72) including an oil removing filter, a regulator, a suction pump, a filter, and a flow meter in this order from the upstream, wherein the exhaust gas introduction line is connected to an exhaust gas pipe at a sample point at an upstream of a junction of the circulation path of the internal combustion engine and that introduces a part of the exhaust gas into the other nondispersive infrared gas analyzer without removing the water, and a temperature adjusting mechanism (81a-c, 82a-c) that keeps a temperature of whole of the intake air introduction line and the exhaust gas introduction line and a temperature of the nondispersive infrared gas analyzers (11, 12) so as not to condense dew and that keeps the temperature of the nondispersive infrared gas analysers (11, 12) higher than the temperature of the intake air introduction line having the fluid equipment and of the exhaust gas introduction line having the fluid equipment. a pair of nondispersive infrared gas analyzers (11, 12) that have a function of correcting a water influence and that are adapted to measure a concentration of CO 2 in a gas containing water, an intake air introduction line having a fluid equipment (21, 31, 41 51,61, 71) including an oil removing filter, a regulator, a suction pump, a filter, and a flow meter in this order from the upstream, wherein the intake air introduction line is connected to an intake air pipe at a sample point at a downstream of a Junction of a circulation path of the internal combustion engine and that Introduces a part of the intake air into one of the nondispersive infrared gas analyzers without removing the water, an exhaust gas introduction line having a fluid equipment (22, 32, 42, 52, 62, 72) including an oil removing filter, a regulator, a suction pump, a filter, and a flow meter in this order from the upstream, wherein the exhaust gas introduction line is connected to an exhaust gas pipe at a sample point at an upstream of a junction of the circulation path of the internal combustion engine and that introduces a part of the exhaust gas into the other nondispersive infrared gas analyzer without removing the water, and a temperature adjusting mechanism (81a-c, 82a-c) that keeps a temperature of whole of the intake air introduction line and the exhaust gas introduction line and a temperature of the nondispersive infrared gas analyzers (11, 12) so as not to condense dew and that keeps the temperature of the nondispersive infrared gas analysers (11, 12) higher than the temperature of the intake air introduction line having the fluid equipment and of the exhaust gas introduction line having the fluid equipment.