A combustor of two-stage combustion type in which fuel is supplied only from primary fuel nozzles for combustion only in primary combustion chamber under the operating condition where gas turbine load is low, and premixed mixture is supplied into a secondary combustion chamber provided on the downstream side of the primary combustion chamber for combustion in both the primary and secondary combustion chambers under the operating condition where the load is high. An air-bleed passageway opens on a downstream side of air holes for intaking secondary combustion air to be mixed with secondary fuel and communicates with the outside of the combustor. The air-bleed passageway is provided with a regulating valve and the secondary combustion air is bled at the minimum point of the load where the secondary fuel is supplied so that the fuel to air ratio of secondary premixed mixture may not be excessively lean, thereby reducing the production of non-burned components.