Attempt was made to find a simple and rapid determination method for rutin in Kaika, a crude drug composed of the flower buds of Sophora japonica L. The reaction conditions of rutin-Al complex, and the correlation between rutin concentration and absorption were statistically examined. The flower buds were extracted with 99% methanol, using a new type extractor, treated with ether to remove quercetin, and diluted with a suitable amount of methanol to prepare a sample solution. To 2cc. of this sample solution, 3cc. of 0.1 mole aqueous solution of aluminum chloride and 5cc. of 1 mole aqueous solution of potassium acetate were added, allowed to stand at a room temperature for two hours, and the absorption (y) of rutin-Al complex was measured by a photoelectric colorimeter, using S-47 filter. The percentage of rutin (x) at 95% confidence limit was calculated from the following equation: x=(0.0612y-0.00072)±0.00049.