A novel diagnostic technique named a “Tracer-Producing LIF technique” which enables 2-dimensional measurement of an internal EGR within an engine cylinder, has been developed. The main feature of this technique is the utilization of a fuel additive that does not itself emit an LIF signal by irradiation of UV-light but whose combustion products radiate strong LIF emissions by UV-light irradiation. Internal EGR behaviors can be measured by observing LIF images that are excited by a UV-laser sheet. Firstly, principles of this technique were confirmed and fuel additives were selected. Then, the “Tracer-Producing LIF technique” was applied to an optically accessible single-cylinder gasoline engine in which the entire pent-roof area can be observed from the side of the engine. The internal EGR behaviors were measured through the entire engine cycle, from intake to exhaust. As a result, gas exchange process between burned gas and fresh air, as well as the influence of the valve timing on the above process, were clarified. It was also confirmed that non-homogeneous internal EGR distribution was still present at the ignition timing. Furthermore, by compensating for the influence of pressure and temperature dependency on LIF intensity, a relative internal EGR concentration measurement has been developed. So evaluation of non-uniformity of the internal EGR in different crank angles has become possible.