A reagent includes a coloring agent allowing a measured solution to develop a color transmitting a region component light for a dissolved-substance concentration measurement selected from three region component lights of red, green, and blue obtained by transmitting a light including a visible light region to the measured solution colored by addition of the reagent to the sample, and dividing a light of the visible light region of a transmitted light thereof into roughly three portions. Based on the transmitted light from the measured solution with the coloring agent added, an absorbance of the region component light for the dissolved-substance concentration measurement, and an absorbance A3 of another region component light resulted only from the coloring agent are calculated. Whether or not the reagent in a necessary quantity is added to the sample can be judged by comparing the absorbance A3 and a standard absorbance A0.