A two-shaft gas turbine is capable of starting premixed combustion without extinguishing a flame. The two-shaft gas turbine includes a combustor and a gas generator controller. The combustor has a premix burner that includes combustion regions in which premixed combustion is to be carried out individually. The gas generator controller controls the combustor. In a method for starting the premixed combustion in the combustor, the gas generator controller selects at least one of the combustion regions in which the premixed combustion is to be carried out, on the basis of a fuel-air ratio, and starts premix combustion in the selected combustion region or separately in each of the selected combustion regions. Further, as the fuel-air ratio is increased, the controller increases the number of the selected region in which the premixed combustion is carried out.