A method of configuring a combustor, which combustor comprises: a combustor external cylinder; a tubular combustor basket installed in the combustor external cylinder; a compressed air channel defined between an inner circumferential surface of the combustor external cylinder and an outer circumferential surface of the combustor basket; and a plurality of fuel nozzles (5,6) installed in the combustor basket, extending approximately in parallel with a central axis of the combustor basket, and arranged to inject fuel for premixing combustion from at least one thereof, wherein the combustor is further arranged such that compressed air flowing along the compressed air channel is approximately reversed in a flow direction at an end of the combustor basket, and is introduced into the plurality of fuel nozzles (5,6), and fuel injection pegs (19,20), each of which is made up of an upstream side fuel injection peg for injecting upstream side fuel and a downstream side fuel injection peg for injecting downstream side fuel, are installed in the compressed air channel at a distance L in the flow direction of the compressed air flowing along the compressed air channel, the method being characterised in that it comprises determining the distance L by calculation based on the following equation: L = d × v / 2 ⁢ π ⁢ f, wherein d is a phase lag between a phase of concentration fluctuation of the upstream side fuel to be injected from the upstream side fuel injection peg and a phase of concentration fluctuation of the downstream side fuel to be injected from the downstream side fuel injection peg, v is a velocity of the compressed air, and f is a frequency of the concentration fluctuation of the upstream side fuel and of the downstream side fuel. a combustor external cylinder; a tubular combustor basket installed in the combustor external cylinder; a compressed air channel defined between an inner circumferential surface of the combustor external cylinder and an outer circumferential surface of the combustor basket; and a plurality of fuel nozzles (5,6) installed in the combustor basket, extending approximately in parallel with a central axis of the combustor basket, and arranged to inject fuel for premixing combustion from at least one thereof, wherein the combustor is further arranged such that compressed air flowing along the compressed air channel is approximately reversed in a flow direction at an end of the combustor basket, and is introduced into the plurality of fuel nozzles (5,6), and fuel injection pegs (19,20), each of which is made up of an upstream side fuel injection peg for injecting upstream side fuel and a downstream side fuel injection peg for injecting downstream side fuel, are installed in the compressed air channel at a distance L in the flow direction of the compressed air flowing along the compressed air channel, the method being characterised in that it comprises determining the distance L by calculation based on the following equation: L = d × v / 2 ⁢ π ⁢ f, wherein d is a phase lag between a phase of concentration fluctuation of the upstream side fuel to be injected from the upstream side fuel injection peg and a phase of concentration fluctuation of the downstream side fuel to be injected from the downstream side fuel injection peg, v is a velocity of the compressed air, and f is a frequency of the concentration fluctuation of the upstream side fuel and of the downstream side fuel.