To provide an engine system of a natural gas engine constructed at low cost and capable of sufficiently purifying exhaust gas without consuming excessive energy and an exhaust emission control method using the same.

The engine system is provided with a gas separation unit 7 separating gas composition in fuel gas supplied from a fuel gas supply opening 5 into light composition containing little non-methane hydrocarbon and heavy composition containing much non-methane hydrocarbon, a first storage tank 9 storing the light composition separated by the gas separation unit 9 and a second storage tank 11 storing the heavy composition separated by the gas composition separation unit 7, a catalyst temperature sensor 6a detecting temperature of a purification catalyst 6, and a switching valve 18 switching fuel gas supplied to the natural gas engine 2 to the light composition or the heavy composition according to a detection result of the catalyst temperature sensor 6a.