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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.