A Concentration measurement device 100 comprises: a measurement cell 4 having a flow path through which a gas flows, a light source 1 for generating incident light to the measurement cell, a photodetector 7 for detecting light emitted from the measurement cell, a pressure sensor 20 for detecting a pressure of the gas in the measurement cell, a temperature sensor 22 for detecting a temperature of the gas in the measurement cell, and an arithmetic circuit 8 for calculating a concentration of the gas based on an output P of the pressure sensor, an output T of the temperature sensor, an output I of the photodetector, and an extinction coefficient α, wherein the arithmetic circuit 8 is configured to calculate the concentration using the extinction coefficient α determined on the basis of the output of the temperature sensor 22.