With progression of so-called shale gas revolution, gas engines are expected as a strong substitute for diesel engines in marine fields, where strict emission regulations have been recently introduced. Thanks to the sulphur-free and low-carbon features of natural gas, gas engines emit much less CO2 and particulate matter than marine diesels burning heavy fuel oil. The premixed lean-burn gas engines, however, suffer two massive flaws. One is abnormal combustion called knocking and the other is a methane slip, which substantially means the unburned methane emitted into exhaust ports.One of the methane slip sources is thought to be flame quenching inside dead volumes around a combustion chamber or inside a boundary layer near a cylinder wall. Only supportive measures like cutdown of crevice volume have been conducted against the unburned methane. The present study proposes novel and essential methane slip reduction for the first time using a gas permeation membrane, of which permeability to oxygen molecules excels the one to nitrogen molecules. The membrane inserted between a main compressor and a charge cooler helps to form uneven oxygen gradient inside a combustion chamber. After the feasibility check of the complicated charging system based on a one-dimensional engine simulator, the potential of the methane slip reduction is successfully examined through CFD simulation with detailed chemical analysis.