A control method for controlling a gas engine of a precombustion chamber type, the gas engine comprising: a main combustion chamber; a precombustion chamber; wherein fuel gas is supplied to the main combustion chamber and the precombustion chamber, a main fuel supply valve for regulating the amount or the flow rate of the fuel toward the main combustion chamber; a precombustion chamber fuel supply valve for regulating the flow amount or the flow rate of the fuel for the precombustion chamber; a spark plug for igniting the air fuel-gas mixture in the precombustion chamber; a charging air pressure/temperature detecting means for measuring the charging air pressures and temperatures; an engine speed sensor for detecting the engine speed; a valve opening duration control means for controlling the valve opening duration regarding the main fuel supply valve; a valve opening duration computing means for computing the valve opening duration regarding the precombustion chamber fuel supply valve; the control method being characterized by comprising: a first step for detecting the charging air pressure and the charging air temperature by the charging air pressure/temperature detecting means and detecting the engine speed by the engine speed sensor; a second step for computing an air excess ratio of the air fuel mixture supplied to the main combustion chamber, based on the charging air pressure, the charging air temperature, the engine speed, and the valve opening duration regarding the main fuel supply valve provided from the valve opening duration control means; wherein the air excess ratio is determined by a following relational expression: λ m = k Ne rpm ⋅ Ps Pa / θ v s ⋅ Ts ° C where k is a constant, Am is the air excess ratio of premixed air fuel mixture in the main chamber; Ne is engine speed; Ps is a pressure of a charging air; θv is a valve opening duration regarding a main fuel supply valve; and, Ts is a temperature of a charging air; a third step for computing the amount of the fuel to be supplied to the precombustion chamber by the air excess ratio, and a fourth step for changing a valve opening period of the precombustion chamber fuel supply valve on the basis of the amount of the fuel. a main combustion chamber; a precombustion chamber; wherein fuel gas is supplied to the main combustion chamber and the precombustion chamber, a main fuel supply valve for regulating the amount or the flow rate of the fuel toward the main combustion chamber; a precombustion chamber fuel supply valve for regulating the flow amount or the flow rate of the fuel for the precombustion chamber; a spark plug for igniting the air fuel-gas mixture in the precombustion chamber; a charging air pressure/temperature detecting means for measuring the charging air pressures and temperatures; an engine speed sensor for detecting the engine speed; a valve opening duration control means for controlling the valve opening duration regarding the main fuel supply valve; a valve opening duration computing means for computing the valve opening duration regarding the precombustion chamber fuel supply valve; a first step for detecting the charging air pressure and the charging air temperature by the charging air pressure/temperature detecting means and detecting the engine speed by the engine speed sensor; a second step for computing an air excess ratio of the air fuel mixture supplied to the main combustion chamber, based on the charging air pressure, the charging air temperature, the engine speed, and the valve opening duration regarding the main fuel supply valve provided from the valve opening duration control means; wherein the air excess ratio is determined by a following relational expression: λ m = k Ne rpm ⋅ Ps Pa / θ v s ⋅ Ts ° C where k is a constant, Am is the air excess ratio of premixed air fuel mixture in the main chamber; Ne is engine speed; Ps is a pressure of a charging air; θv is a valve opening duration regarding a main fuel supply valve; and, Ts is a temperature of a charging air; a third step for computing the amount of the fuel to be supplied to the precombustion chamber by the air excess ratio, and a fourth step for changing a valve opening period of the precombustion chamber fuel supply valve on the basis of the amount of the fuel.