The concentration measurement device 100 includes an electric unit 20 having a light source 22 and a photodetector 24, a fluid unit 10 having a measurement cell 1, a first light-transmission member 11 for transmitting light from the light source to the measurement cell, a second light transmission member 12 for transmitting light from the measurement cell to the photodetector, a lens 3A provided in the fluid unit, the lens 3A being arranged such that light from the first light transmission member is to be incident on the first position and light is to be emitted from the second position to the second light transmission member, a pressure sensor 5 for measuring pressure of fluid flowing through the measurement cell, and an arithmetic circuit 28 for detecting concentration of the fluid flowing through the measurement cell, the arithmetic circuit being configured to calculate the fluid concentration based on the output of the photodetector and a correction factor related to the pressure output by the pressure sensor and the concentration of fluid in order to reduce the measurement error due to the refractive index of the fluid.