The following terms are necessary for a theoretical quantitative study of gas leakage through the piston-rings of internal combustion engine : (1) the discharge coefficient through the narrowest gas passage, (2) the temperature of gas in the space between two rings. So at first the former is measured by letting a high temperature compressed air flow out through a gap of piston-ring set in the experimental apparatus. Next, the latter is studied using the apparatus provided with a stationary piston and a reciprocating cylinder. As a result, the following conclusions are obtained : (a) the discharge coefficients are 0.8∼0.9 and its mean value is about 0.86 when the states of leakage gas are near to those of an actual working engine, (b) the temperature of gas flowing out around the piston-rings almost is equal to that of gas on the surface of piston.