For the purpose of analyzing the combustion process in a spark ignition engine, it is necessary to know the combustion temperature in the combustion chamber. The author has developed a new technique of measuring the combustion temperature using the sodium line reversal method. By this new technique, graduation of temperature is possible to the output of flame radiation from the combustion chamber. The distribution of flame temperature in the cylinder could be measured by this method even near the cylinder wall. For the purpose of measuring the flame temperature in the commercial engine cylinder, the compact sensor with prisms and optical fibers was developed. Measured value of these techniques are discussed for the various operating conditions of the engine.