The history of the study to apply the ion current to gasoline engine control is so long, and many papers on modeling and application have been published so far. Especially, the study on modeling has been advanced, and it has become possible to estimate the ion current. And in the area of application, many papers have proposed the detection method of engine knock, pre-ignition, combustion stability and so on. However, there few results the proposed methods were put to the production engine.This paper proposes the internal exhaust gas recirculation (EGR) control system based on ion current which enables to set the limit value of internal EGR rate, so as to extract the best fuel efficiency of engine itself. This system has the following features which can contribute to putting to commercial use. 1To detect the engine stability accurately, the original parameter considering the following two items was developed:iIn case of applying the gap of ignition plug as Ion probe, the detection accuracy of early combustion phase is higher than the one of later phase.iiThe gasoline engine has the characteristic which alternates stable and unstable combustion, when the fluctuation of indicated mean effective pressure (IMEP) becomes larger.2To control the EGR rate to limit value quickly by using the above combustion fluctuation parameter, the robust regulator not including integral function was applied. And also, the gain of regulator was designed based on control theory.