A gas turbine combustor is capable of reducing NOx over a whole load range of a gas turbine. The combustor comprises two burner systems. It is possible to independently control a flow rate of combustion air supplied to a burner of the respective systems and a flow rate of bypass air jetted from through a bypass air port. The air supplied to the burners is mixed with fuel in a mixing chamber and then ejected into a combustion chamber so as to form a flame. The amount of bypass air is controlled in accordance with an operation load, a humidity of air and a heating value of fuel. It is therefore possible to prevent any blow off and reduce NOx over the whole load range of the gas turbine.