A fluid concentration measuring device which is configured to measure concentration of a fluid flowing through a resin tube having a light-transmissive tube wall being deformable in a diametrical direction thereof, the device comprising: a light source which is configured to supply light into the resin tube from a light supply part on a surface of the resin tube; a light receiving element which is configured to receive the light, which has been supplied and passed through the tube wall of the resin tube and the fluid inside the resin tube, at a light receiving part located on an opposite side in a diametrical direction of the resin tube relative to the light supply part, and output a signal indicating intensity of the light; light path distance setting means (4,18) which is configured to set a plurality of light path distances between the light supply part and the light receiving part by compressively deforming the resin tube having the tube wall, in the diametrical direction, while holding the resin tube; and fluid concentration output means which, from the light intensity at the light receiving part located at each of the plurality of light path distances, is configured to obtain a plurality of relational expressions, which indicate relation between the light intensity and the fluid concentration when the light from the light supply part is received by the light receiving part over each of the light path distances, based on the Beer-Lambert law, and obtain the fluid concentration from the light intensity at the light receiving part based on the relational expressions for the plurality of light path distances and output the fluid concentration, wherein the light path distance setting means has a plurality of pairs (4S, 4L) of light supply part and light receiving part having different intervals therebetween, and is configured to change the light path distance by selectively using the pairs of light supply part and light receiving part; and/or the light path distance setting means is configured to change the light path distance between the light supply part and the light receiving part by varying the interval between the light supply part and the light receiving part. a light source which is configured to supply light into the resin tube from a light supply part on a surface of the resin tube; a light receiving element which is configured to receive the light, which has been supplied and passed through the tube wall of the resin tube and the fluid inside the resin tube, at a light receiving part located on an opposite side in a diametrical direction of the resin tube relative to the light supply part, and output a signal indicating intensity of the light; light path distance setting means (4,18) which is configured to set a plurality of light path distances between the light supply part and the light receiving part by compressively deforming the resin tube having the tube wall, in the diametrical direction, while holding the resin tube; and fluid concentration output means which, from the light intensity at the light receiving part located at each of the plurality of light path distances, is configured to obtain a plurality of relational expressions, which indicate relation between the light intensity and the fluid concentration when the light from the light supply part is received by the light receiving part over each of the light path distances, based on the Beer-Lambert law, and obtain the fluid concentration from the light intensity at the light receiving part based on the relational expressions for the plurality of light path distances and output the fluid concentration, wherein the light path distance setting means has a plurality of pairs (4S, 4L) of light supply part and light receiving part having different intervals therebetween, and is configured to change the light path distance by selectively using the pairs of light supply part and light receiving part; and/or the light path distance setting means is configured to change the light path distance between the light supply part and the light receiving part by varying the interval between the light supply part and the light receiving part.