A gas turbine combustor comprising: a combustor liner as an inner duct; a flow sleeve as an outer duct, in which the combustor liner is provided; and an annular flow passage formed between the combustor liner and the flow sleeve, through which compressed air flows, characterized in that the flow sleeve includes a narrowing member formed on an inner wall of the flow sleeve, the narrowing member protruding toward the combustor liner; the combustor liner includes an annular protruding portion annularly formed on an outer wall of the combustor liner, the annular protruding portion protruding toward the flow sleeve; the narrowing member includes an internal-diameter changing portion and an internal-diameter reducing portion; the internal-diameter changing portion is a plane diagonally connected to the flow sleeve to gradually approach the combustor liner as the internal-diameter changing portion extends in a flow direction of the compressed air; and the internal-diameter reducing portion is a plane disposed at a downstream side of the internal-diameter changing portion in the flow direction of the compressed air, connected to the internal-diameter changing portion, and extending along the flow direction of the compressed air; characterized in that the annular protruding portion is located at a position on the outer wall of the combustion liner, the position facing a connection position between the flow sleeve and the internal-diameter changing portion or at an upstream side of the position facing the connection position in the flow direction of the compressed air such that a length of a downstream separation vortex generated by the protruding portion is reduced in the flow direction of the compressed air due to the flow velocity vector of the compressed air which is bent by the internal-diameter changing portion. a combustor liner as an inner duct; a flow sleeve as an outer duct, in which the combustor liner is provided; and an annular flow passage formed between the combustor liner and the flow sleeve, through which compressed air flows, characterized in that the flow sleeve includes a narrowing member formed on an inner wall of the flow sleeve, the narrowing member protruding toward the combustor liner; the combustor liner includes an annular protruding portion annularly formed on an outer wall of the combustor liner, the annular protruding portion protruding toward the flow sleeve; the narrowing member includes an internal-diameter changing portion and an internal-diameter reducing portion; the internal-diameter changing portion is a plane diagonally connected to the flow sleeve to gradually approach the combustor liner as the internal-diameter changing portion extends in a flow direction of the compressed air; and the internal-diameter reducing portion is a plane disposed at a downstream side of the internal-diameter changing portion in the flow direction of the compressed air, connected to the internal-diameter changing portion, and extending along the flow direction of the compressed air; characterized in that the annular protruding portion is located at a position on the outer wall of the combustion liner, the position facing a connection position between the flow sleeve and the internal-diameter changing portion or at an upstream side of the position facing the connection position in the flow direction of the compressed air such that a length of a downstream separation vortex generated by the protruding portion is reduced in the flow direction of the compressed air due to the flow velocity vector of the compressed air which is bent by the internal-diameter changing portion.