We examined colorimetric methods for the determination of calcium and magnesium in brewing water. The following results were found by the modification of these methods. (1) In the Yanagisawa's method the measured value of magnesium was lower than the theoretical value, but for calcium there was little error and the result was approximately accurate. (2) By the titan yellow method magnesium could be determined in considerable accuracy without any change of color with time and without influence of interfering ions when added with stabilizing agents (starch and sucrose). (3) The determination of calcium by the phosphate method showed some change of transmittance percentage with time, but accurate results were obtained without large change of color and without influence of interfering action when determined in 5 minutes. Among the three methods the titan yellow and phosphate methods were recognized to be applicable for the determination of hardness in brewing water rapidly and accurately by using a photoelectric colorimeter.