It has been shown that lean burn is effective for improving the thermal efficiency of gasoline SI engines. This happens because the reduction of heat loss by decrease of flame temperature. On the other hand, the fuel dilution of the premixed gas makes the combustion speed low, and cycle-to-cycle variations of combustion are increased by excessive dilution, it is difficult to increase the thermal efficiency of the gasoline SI engine. Influence of ignition by spark discharge is considered as a factor of combustion variation, and it is necessary to understand the effects of spark discharge characteristics on the lean combustion process. Spark discharge in the SI engines supplies energy to the premixed-gas via a discharge channel in the spark plug gap which ignites the premixed-gas. The discharge channel is elongated by in-cylinder gas flow and its behavior varies in each cycles. When the discharge channel is elongated, the premixed-gas on the discharge channel increases and the gap resistance increases. As a result, the pattern of the discharge energy supply to the premixed gas by the spark discharge also changes in each cycle. It may affect the lean-burn combustion process. However, the relationship between spark discharge and combustion has not been clarified.In this study, using an ignition system with 10 ignition coils and an adapter for intensify the tumble flow, we photograph the behavior of the discharge channel with a visualization engine. The relationship between discharge channel length and gap resistance are clarified. Then combustion experiments were carried out, discharge current, discharge voltage and in-cylinder gas pressure were measured at the same time. We investigated the influence of gap resistance on the combustion phaseAs a result, there was a correlation between the discharge channel area and the gap resistance. It is possible to estimate the spark discharge channel area from the gap resistance, by measuring the discharge voltage and the discharge current without using the visualization device. When the average gap resistance was more than 45 kΩ , CA 5 became 0 deg.ATDC or less except for 1 cycle. Therefore, it was revealed that combustion was promoted when the discharge channel was stretched and the gap resistance was large.