The thermal efficiency of gas engines have been improved considerably in the last decade and now some of gas engines has achieved higher efficiency than diesel engines by introducing the state of the arts technologies. But at the same time, the abnormal combustion with the high peak firing pressure has been seen in recent years especially on high BMEP engines. This combustion has the unique characteristics such that it occurs temporally with the limited crank angle of advanced timing and the retarded spark ignition timing makes it worse. As the results of researches including the visualization tests on the single cylinder engine, this combustion was confirmed to be the kind of pre-ignition caused by the auto ignition of in-cylinder lubricating oil. And it was found to be one of the causes of the cyclic variation of peak firing pressure on premixed combustion gas engine. The lubricating oil can be ignited in the cylinder near the end of compression stroke where the temperature in cylinder is high enough for fuel oil to ignite. This is a natural phenomenon because the auto ignition temperature of lubricating oil is same level as that of fuel oil. But this phenomenon has not been considered so deeply so far because it has not caused serious combustion problems. But from now, more careful design by adding the view of the abnormal combustion caused by lubricating oil will be required for the future engines having the higher BEMP and thermal efficiency. Especially in case of gas engine ignited by fuel oil, such as Micro Pilot and Dual Fuel engine, the stable combustion window where combustion can be initiated and controlled not by lubricating oil but by fuel oil will become narrow. In this paper, the characteristics of this abnormal combustion on production engines and the visualization data of combustions measured on a test engine are introduced and then the consideration about the mechanism and the analysis of this phenomenon are described in detail.