It is proposed that a premixed-compression-ignition natural gas engine could be realized by gaseous formaldehyde addition into the intake fuel/air mixtures. Large-scale natural gas engines could not be realized even by prechamber-type, spark-ignition engines due to knocking and/or flame propagation problems, nor by diesel operations due to poor ignition characteristics of the fuel. The formaldehyde addition would show a strong promoting effect for lean mixture ignition of natural gas. When suitable amount of formaldehyde is added into the intake fuel/air mixture the piston compression ignition will occur adequately near the top dead center. Confirmation experiments of the proposed concept were carried out herewith using a single-cylinder engine and a commercial natural gas 13A as fuel. This procedure enabled us to control ignition timing most adequate for engine operations and to obtain ignition or hot-flame explosion even of the fuel/air mixtures nonflammable through a simple piston compression. The appropriate amounts of formaldehyde acts efficaciously as the ignition-promoting additive for the methane-based gaseous fuels which are weak in cool-flame generation during the preflame induction period. Almost no NOx nor soot emission is emitted as well as the previously proposed lean high/low octane two- fuel premixed compression-ignition engine operation using other fuels such as propane, iso-octane and n-heptane.