A gas turbine combustor adapted for combusting a fuel together with a compressed air supplied from a compressor and supplying a combustion gas to a turbine, comprising: a main burner provided to a head portion of a combustor cylinder constituting a combustion chamber; and a pre-mixing type supplemental burner provided to a downstream portion of the combustor cylinder relative to the main burner and extending through a circumferential wall of the combustor cylinder, wherein the supplemental burner comprises: an introducing passage configured to deflect a part of the compressed air radially inward with respect to the combustor cylinder, the compressed air flowing from an air passage formed between the circumferential wall of the combustor cylinder and a housing surrounding the circumferential wall toward the head portion of the combustor cylinder, and introduce the compressed air into the combustor cylinder; and a fuel nozzle configured to supply the fuel from a plurality of fuel injection holes to the compressed air which is introduced into the introducing passage so as to produce a pre-mixed gas in the introducing passage; an annular inlet port constituting an inlet of the introducing passage; and a plurality of guide pieces provided to the annular inlet port and configured to guide the compressed air toward a center of the inlet port, and characterised in that: the fuel nozzle includes a nozzle plate constituting a head of the introducing passage, the fuel injection holes being provided in the nozzle plate such that the fuel is supplied into the introducing passage through the fuel injection holes and a space between each adjacent pair of the guide pieces. a main burner provided to a head portion of a combustor cylinder constituting a combustion chamber; and a pre-mixing type supplemental burner provided to a downstream portion of the combustor cylinder relative to the main burner and extending through a circumferential wall of the combustor cylinder, wherein the supplemental burner comprises: an introducing passage configured to deflect a part of the compressed air radially inward with respect to the combustor cylinder, the compressed air flowing from an air passage formed between the circumferential wall of the combustor cylinder and a housing surrounding the circumferential wall toward the head portion of the combustor cylinder, and introduce the compressed air into the combustor cylinder; and a fuel nozzle configured to supply the fuel from a plurality of fuel injection holes to the compressed air which is introduced into the introducing passage so as to produce a pre-mixed gas in the introducing passage; an annular inlet port constituting an inlet of the introducing passage; and a plurality of guide pieces provided to the annular inlet port and configured to guide the compressed air toward a center of the inlet port, and characterised in that: the fuel nozzle includes a nozzle plate constituting a head of the introducing passage, the fuel injection holes being provided in the nozzle plate such that the fuel is supplied into the introducing passage through the fuel injection holes and a space between each adjacent pair of the guide pieces. an introducing passage configured to deflect a part of the compressed air radially inward with respect to the combustor cylinder, the compressed air flowing from an air passage formed between the circumferential wall of the combustor cylinder and a housing surrounding the circumferential wall toward the head portion of the combustor cylinder, and introduce the compressed air into the combustor cylinder; and a fuel nozzle configured to supply the fuel from a plurality of fuel injection holes to the compressed air which is introduced into the introducing passage so as to produce a pre-mixed gas in the introducing passage; an annular inlet port constituting an inlet of the introducing passage; and a plurality of guide pieces provided to the annular inlet port and configured to guide the compressed air toward a center of the inlet port, and characterised in that: the fuel nozzle includes a nozzle plate constituting a head of the introducing passage, the fuel injection holes being provided in the nozzle plate such that the fuel is supplied into the introducing passage through the fuel injection holes and a space between each adjacent pair of the guide pieces. the fuel nozzle includes a nozzle plate constituting a head of the introducing passage, the fuel injection holes being provided in the nozzle plate such that the fuel is supplied into the introducing passage through the fuel injection holes and a space between each adjacent pair of the guide pieces.