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In a qualitative test for urine protein by a test paper based on a protein error of tetrabromophenol blue, how the color development changes was examined by experiments and calculations when a pH change in the reaction mixture occurs in immersing a test paper in a urine sample whose pH may vary approximately from as low as 4.6 to as high as 7.8. In an experiment using bromophenol blue, the color development increased due to an increase in the pH of the reaction mixture, and the relative color intensity was closely associated with the magnitude of the pH change and the concentrations of the dye, buffer and protein. That is, even if the pH change by urine samples was equal, the relative color intensity differed with the concentrations of the dye, buffer and protein. It increased with an increase in the dye and buffer concentrations, and with a decrease in the protein concentration. These experimental results coincided well with those obtained by the calculation. It is important that the detection of a urine sample with a low protein concentration is carried out by using a test paper in which the dye concentration is low in order to suppress a false-positive result.