The authors of this paper studied on the tracer gas method which had already been suggested for application in measuring the trapping efficiency of two stroke cycle engines. And in consequence, the superiority in both accuracy and reproducibility of measurement under the following conditions was recognized : The use of anhydrous monomethylamine as the tracer gas ; gas concentration being 0.1 to 0.3% ; the use of 10 cc of 0.02 normal solution of sulfuric acid as the absorbing liquid ; a mixture of bromcresol-green and methyl-red as the neutralization reagent. The type of sampling tube, the gas sampling points and the gas sampling apparatus are also selected. The accuracy are compared with the results obtained by the gas sampling valve method. Under these conditions, the errors are within the range of ±2%.