In a combustor of a gas turbine which has a pilot nozzle being installed to the center of the axis of a combustor basket and a plurality of main nozzles being installed to the vicinity of the pilot nozzle and provided with a premixing tool on the outer circumference thereof, wherein, fuel being injected as air-fuel pre-mixture from the main nozzle into the interior of a transition piece forming a combustion chamber downstream of the combustor basket is ignited by diffusion flame being generated by the pilot nozzle in the transition piece so as to generate a premixed flame in the transition piece, wherein combustion is performed by a part of the plurality of main nozzles from start-up until a predetermined load rate and then performed by adding the remaining portion of the plurality of main nozzles when the predetermined load rate is exceeded.