A fluid concentration measuring device measures the concentration of a fluid flowing through a duct having a light-transmissive, deformable duct wall. The device includes a light source; a light receiving element which receives the light, which passed through the wall of the duct and the fluid inside the duct, and outputs a signal indicating the intensity of the light; light path distance setting means and fluid concentration output means which obtains a plurality of relational expressions, which indicate the 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, and obtains 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 outputs the fluid concentration.