The determination of the concentration of inflammable anesthetic gases is the most important for the precaution against ignition. A simple method using an interferometer and detector has been found by author for the determination of the concentration of ether gas and cyclopropane. 1) The concentration of ether gas in the air was measured by using Haber-Löwe's interferometer. One percent of ether gas in the air corresponded to the graduation of 77.0 in this interferometer and its lowest limit of measurement was 0.013%. 2) For the determination of the concentration of cyclopropane in the air, the HaberLöwe's interferometer was also used. One percent of cyclopropane in the air corresponded to the graduation of 42.7 and its lowest limit of measurement was 0.025%. 3) As coloring of Hoolamite detector and NBS indicator was caused by cyclopropane and the coloring time was also influenced by its concentration, the relation between the coloring time and the concentration was studied, thus the concentration of cyclopropane was able to be measured. The extent of the concentration measurable by this method was between 0.01% and 5.0%.