A concentration measuring module for measuring concentrations of two constituents contained in a fluid simultaneously, comprising: a housing configured to house the fluid as irradiated body, the housing being provided with an irradiated body inlet through which the irradiated body can be introduced into the housing and an irradiated body outlet through which the irradiated body is discharged from the housing; a light source configured to emit light into the housing; a first light receiving unit configured to have sensitivity to a wavelength of output light of the light source and receive light radiated from inside the housing; and a second light receiving unit configured to have sensitivity to a longer wavelength than the first light receiving unit and receive light radiated from inside the housing, wherein the light source and the first light receiving unit are arranged so that a light emitting surface of the light source faces a light receiving surface of the first light receiving unit, the light source, the first light receiving unit, and the second light receiving unit are arranged to have a positional relationship in which a normal to a light receiving surface of the second light receiving unit is orthogonal to a line segment corresponding to the inside of the housing, of a line through the light source and the first light receiving unit, and a length X of the line segment corresponding to the inside dimension of the housing, of the line through the light source and the first light receiving unit, and a length Y of a line segment corresponding to the inside dimension of the housing, of a line including the normal to the light receiving surface of the second light receiving unit, characterised in that X and Y satisfy 2√3 < Y/X < 50. a housing configured to house the fluid as irradiated body, the housing being provided with an irradiated body inlet through which the irradiated body can be introduced into the housing and an irradiated body outlet through which the irradiated body is discharged from the housing; a light source configured to emit light into the housing; a first light receiving unit configured to have sensitivity to a wavelength of output light of the light source and receive light radiated from inside the housing; and a second light receiving unit configured to have sensitivity to a longer wavelength than the first light receiving unit and receive light radiated from inside the housing, wherein the light source and the first light receiving unit are arranged so that a light emitting surface of the light source faces a light receiving surface of the first light receiving unit, the light source, the first light receiving unit, and the second light receiving unit are arranged to have a positional relationship in which a normal to a light receiving surface of the second light receiving unit is orthogonal to a line segment corresponding to the inside of the housing, of a line through the light source and the first light receiving unit, and a length X of the line segment corresponding to the inside dimension of the housing, of the line through the light source and the first light receiving unit, and a length Y of a line segment corresponding to the inside dimension of the housing, of a line including the normal to the light receiving surface of the second light receiving unit, characterised in that X and Y satisfy 2√3 < Y/X < 50.