Concentration measurement device

A concentration measurement device comprising:

• a measurement cell having a flow path for flowing a fluid to be measured;
• a light source for emitting incident light to the measurement cell;
• a photodetector for detecting light emitted from the measurement cell;
• and an arithmetic circuit for calculating absorbance and concentration of the fluid to be measured on the basis of an output from the photodetector, wherein
• the measurement cell includes a cell body, a window portion fixed to the cell body so as to contact the flow path, and a reflecting member disposed opposite to the window portion across the flow path, the reflecting member reflecting light incident on the measurement cell through the window portion from the light source and reflecting the light from the measurement cell through the window portion,
• the window portion is fixed to the cell body by a window holding member via a gasket, surfaces of the window portion being disposed so as to be inclined at an inclination angle of equal to or greater than 1° and equal to or less than 5° with respect to a surface orthogonal to the central axis of the flow path, and
• the window holding member includes a lower surface for pressing the window portion, formed non-parallel with the respect to an upper surface facing the lower surface, and a collimator connected to the light source is fixed on the upper surface of the window holding member so that the central axis of the collimator is parallel to the central axis of the flow path.