A gas turbine combustor includes a combustion cylinder and a premixing tube. A protruding wall of the tube has an inner diameter enlarging portion with a chamfer at an open end thereof. A swirl flow in the tube is guided outward along the chamfer, so that the cross-section area of a straight flow in the tube is expanded to have a lower flow velocity, and the distance between a flame in the cylinder and the enlarging portion is shortened. When using the fuel with high heat radiation upon combustion, the enlarging portion is replaced with the one having a smaller chamfer. The cross-section area of the straight flow is reduced to increase the flow velocity, and the lift distance becomes longer. Therefore, the damage to the combustion cylinder owing to the intensified heat radiation is prevented. The flame lift position retained in the cylinder is easily set and changed.