A concentration measuring device 100 comprises: a measurement cell 4 having a flow path, a light source 1, a photodetector 7 for detecting light emitted from the measurement cell, and an arithmetic circuit 8 for calculating light absorbance and concentration of a fluid to be measured on the basis of an output of the photodetector, the measurement cell includes a cell body, a window portion 3 fixed to the cell body so as to contact the flow path, and a reflective member 5 for reflecting light incident on the measurement cell through the window portion, the window portion is fixed to the cell body 40 by a window holding member 30 via a gasket 15, an annular sealing protrusion 15a is provided on a first surface of the gasket for supporting the window portion, and an annular sealing protrusion 42a is also provided on a support surface 42 of the cell body for supporting the second surface opposite to the first surface of the gasket.