A method to control a gas engine system is disclosed, whereby the engine can be operated with an air-fuel-ratio controlled with high precision, even in using a low calorific fuel-gas that is prone to vary in calorific value; the engine system includes: a first gas line toward each cylinder via a first gas valve from a gas supply source line, the first gas valve regulating flow rates of the fuel-gas through a gas compressor on the line; a second gas line toward suction air, the line being branched from the gas supply source line and the line being provided with a gas air mixer and a second gas valve on the line. In the case when the fuel-gas is of a low calorific value or where the output of the engine is high, a part of the fuel-gas is supplied to the engine through the first and second lines. The mixer forms a mixture of a prescribed mixing ratio that is leaner than a lower limit of flammability of the fuel-gas, while the remaining fuel-gas through the first line is supplied to the mixture just before each cylinder so that a final prescribed air fuel ratio is formed. In the case when the calorific value of the fuel-gas is high, or of high engine output, the whole fuel-gas can be supplied only through the first gas line.