An oxidizing reagent consisting of the mixture of 2.5 g of potassium dichromate, 10 g of silver sulfate, 600 ml of sulfuric acid and 400 ml of phosphoric acid was recommended to use for the simple and rapid determination of COD of various waste waters. A 10-ml portion of sample water was put into a 50-ml Kjeldahl flask with 0.3 g of mercury (II) sulfate and several pieces of boiling stone, and 25 ml of the reagent-mixture was added. The solution was boiled at 160°C for 30 min, setting a cooling finger inside the neck of the flask. The contents of the flask was transferred into a 50-ml measuring flask and was diluted with water to the mark. The absorbance of the solution was measured at the wavelength of 630nm with a 50-mm glass cell. Beer's law was obeyed over the range of 01000 ppm COD. The calibration curves using glycine, glucose, ethanol and starch, as the reference, gave their slope of 7.05 × 10^-4 as the mean. Therefore, the COD value can be calculated from the following equation : COD =absorbance/ 7.05 × 10^-4. An appropriate volume ratio of sulfuric acid to phosphoric acid for the decomposition of glycine was found to be 3 : 2. The reaction time could be reduced to about one fourth of JIS (COD_cr) method. The analytical results obtained for several kinds of industrial waste water are in good agreement with those obtained by JIS (COD_cr) method.