62 is set so that the absorbances at the wavelengths u03bb 1 and u03bb 2 may become equal i e the difference u03b4d of the absorbances at u03bb 1 and u03bb 2 may become zero in this case it is convenient that the value of lower absorbance is made equal to the value of higher absorbance by the coefficient multiplier in this way the relations of b u03bb 2 b u03bb 1 b 39 u03bb 2 b 39 u03bb 1 k 1 k 2 are obtained after the above setting has been done the mixture c is measured whereby even when the concentrations of the coexistent materials b and b 39 change the material to be measured a can be measured without the influences thereof further the output is converted into the concentration by the coefficient k 3 of the third coefficient multiplier 81 and can therefore be directly read concrete examples of measurements with the apparatus of the embodiment of this invention will be explained hereunder fig5 is a graph showing the spectrum absorbing characteristic of an edible coloring matter pholoxine alone fig6 illustrates the results of measurements in the case of considering the coloring matter pholoxine as the coexistent material and employing two sorts of combinations of the two wavelengths i e the combination of wavelengths 480 nm and 540 nm and the combination of wavelengths 510 nm and 560 nm as understood from.