A combustor of a gas turbine comprising: at least a pair of premixed gas generators (33,34) including an inner premixed gas generator and an outer premixed gas generator for producing a combustion flame; an inner cylinder that has therein the premixed gas generators (33,34); and a transition piece for connecting the inner cylinder to an inlet of a turbine, wherein the outer premixed gas generator is placed outward of the inner premixed gas generator in the radial direction of the combustor and surrounds an outer circumference of the inner premixed gas generator, the inner premixed gas generator and the outer premixed gas generator are placed with a cylindrical swirler ring interposed therebetween, a premixed gas generator outlet of the inner premixed gas generator is located upstream of a premixed gas generator outlet of the outer premixed gas generator in the axial direction of the combustor, the premixed gas generator outlets (333,343) of the premixed gas generators (33,34) respectively have a circumferential wall surface, and a part of the circumferential wall surface which is located outward in a radial direction of the combustor, extends further in an axial direction of the combustion flame than a part of the circumferential wall surface which is located inward in the radial direction of the combustor, an inner circumferential surface of the swirler ring forms the part of the circumferential wall surface of the inner premixed gas generator which is located inward in the radial direction of the combustor and which extends steplessly from the premixed gas generator outlet of the inner premixed gas generator in the axial direction of the transition piece up to a step constituted by the swirler ring such that, in operation, the distal end of the combustion flame produced in the inner premixed gas generator is located at an edge of the swirler ring, and an inner circumferential wall surface of the transition piece forms the part of the circumferential wall surface of the outer premixed gas generator which is located outward in the radial direction of the combustor, wherein the inner circumferential wall surface of the transition piece extends steplessly from the premixed gas generator outlet of the outer premixed gas generator in the axial direction of the transition piece up to a step formed by widening the wall surface of the transition piece in a stepped manner and which is provided at a position slightly upstream of a tip of a combustion flame which is produced, in operation, in the outer premixed gas generator. at least a pair of premixed gas generators (33,34) including an inner premixed gas generator and an outer premixed gas generator for producing a combustion flame; an inner cylinder that has therein the premixed gas generators (33,34); and a transition piece for connecting the inner cylinder to an inlet of a turbine, wherein the outer premixed gas generator is placed outward of the inner premixed gas generator in the radial direction of the combustor and surrounds an outer circumference of the inner premixed gas generator, the inner premixed gas generator and the outer premixed gas generator are placed with a cylindrical swirler ring interposed therebetween, a premixed gas generator outlet of the inner premixed gas generator is located upstream of a premixed gas generator outlet of the outer premixed gas generator in the axial direction of the combustor, the premixed gas generator outlets (333,343) of the premixed gas generators (33,34) respectively have a circumferential wall surface, and a part of the circumferential wall surface which is located outward in a radial direction of the combustor, extends further in an axial direction of the combustion flame than a part of the circumferential wall surface which is located inward in the radial direction of the combustor, an inner circumferential surface of the swirler ring forms the part of the circumferential wall surface of the inner premixed gas generator which is located inward in the radial direction of the combustor and which extends steplessly from the premixed gas generator outlet of the inner premixed gas generator in the axial direction of the transition piece up to a step constituted by the swirler ring such that, in operation, the distal end of the combustion flame produced in the inner premixed gas generator is located at an edge of the swirler ring, and an inner circumferential wall surface of the transition piece forms the part of the circumferential wall surface of the outer premixed gas generator which is located outward in the radial direction of the combustor, wherein the inner circumferential wall surface of the transition piece extends steplessly from the premixed gas generator outlet of the outer premixed gas generator in the axial direction of the transition piece up to a step formed by widening the wall surface of the transition piece in a stepped manner and which is provided at a position slightly upstream of a tip of a combustion flame which is produced, in operation, in the outer premixed gas generator.