Widely used for spot analysis of breath alcohol of drivers by the Japanese Police Authority, the detector tube is described as capable of measuring breath alcohol from 0.10 mg/1 to 1.20 mg/1 with an accuracy of less than + 10 percent in relative error. The internal operation of the tube, which utilizes acidic chromic trioxide, is discussed. The concurrent use of an alcohol concentration chart, which is calibrated to the length of the stain visible on the tube after a breath alcohol test, is graphically illustrated. The results of comparing the accuracy of the detector tube and gas chromatography methods for detecting breath alcohol are shown. The paper reports a high correlation in accuracy between the two methods and a minimal effect of other gases on the detector tube method. The development of the double-type bag in order to preserve the level of alcohol concentration in the breath during the measurement process is illustrated. Standards used for detector tube measurement of breath alcohol by the Japanese police are identified. Six figures, two tables, and four references are provided.