The influences of humidity and temperature, which were considered to be the operating room condition, upon the explosiveness of ether gas and cyclopropane were studied concerning the minimum ignition energy. The results were as follows: 1) The curve of minimum ignition energy of the mixed gases of ether or cyclopropane with the air was estimated by the intensity of primary electric current of an induction coil, which was used for spark discharge. 2) When the temperature of the mixed gases of ether or cyclopropane with the air raised, the minimum ignition energy became less, and more explosive. On this result, the most optimum temperature of the operating room, for the precaution against ignition and also for clinical necessity, was quantatively considered to be from 18°C to 24°C. 3) The minimum ignition energy was less in case of higher humidity of the mixed gases of ether or cyclopropane with the air than in case of lower humidity, namely it became more explosive. To keep the humidity for the purpose of preventing against the generation of the static electricity is more important than the problem of the minimum ignition energy, so that the relative humidity of 60% is to be the most optimum humidity of the operating room for precaution against explosion.