Concentration measurement device

A concentration measurement device configured to measure a concentration of a gas flowing through a junction block connected to a plurality of gas supply lines, comprising:

• a light source for generating light incident to a flow path formed in the junction block;
• a photodetector for receiving light emitted from the flow path;
• and
• an arithmetic control circuit for calculating the concentration of the gas flowing through the flow path based on an output of the photodetector,
• wherein at least one of a translucent incident window for making light from the light source incident to the flow path and a translucent emitting window for emitting light passing through the flow path is sealed and fixed to the junction block,
• wherein the junction block is an outlet side flow path block fixed on a base plate of an integrated unit in which the plurality of gas supply lines are formed on the base plate,
• wherein the junction block includes a plurality of sub-flow paths respectively connected to the plurality of gas supply lines;
• and a main flow path connected to the plurality of sub-flow paths, and
• wherein the main flow path is formed by a through hole extending along a longitudinal direction of the junction block, and each of the plurality of sub-flow paths is formed by a hole extending in a direction intersecting with the main flow path and provided so as to reach the main flow path from a surface of the junction block.