A fuel injector comprising: an upstream support plate configured to introduce a fuel gas from a first end side into an inner side in a direction of an axis and having a shape of a tapered cylinder whose diameter gradually enlarges toward a second end side in the direction of the axis; a downstream support plate provided to intersect the axis, disposed at the second end side of the upstream support plate in the direction of the axis, and configured to define a plenum at an inner side along with the upstream support plate; and a plurality of premixing tubes (13;23) provided to extend in the direction of the axis to be supported on the upstream and downstream support plates (11,12) and configured to introduce air from the first end side in the direction of the axis, wherein the plurality of premixing tubes (13;23) are disposed in rows shaped as a plurality of circles whose radial dimensions centered on the axis are different from each other, the neighboring premixing tubes (13;23) disposed in the same row are disposed equidistantly from each other in a circumferential direction, and portions of the premixing tubes (13;23) which are located in the plenum are formed with fuel introduction holes penetrating the premixing tubes (13;23) from outside to inside such that the fuel gas supplied from the plenum into the premixing tubes (13;23) via the fuel introduction holes in operation is mixed with the air in the premixing tubes (13;23) and is injected from second end sides of the premixing tubes (13;23) in the direction of the axis, characterized in that a length of the plenum in each row in the direction of the axis is set such that a flow velocity (v) of the fuel gas circulating in circumferential spaces between the plurality of premixing tubes (13;23) in a radial direction is constant. an upstream support plate configured to introduce a fuel gas from a first end side into an inner side in a direction of an axis and having a shape of a tapered cylinder whose diameter gradually enlarges toward a second end side in the direction of the axis; a downstream support plate provided to intersect the axis, disposed at the second end side of the upstream support plate in the direction of the axis, and configured to define a plenum at an inner side along with the upstream support plate; and a plurality of premixing tubes (13;23) provided to extend in the direction of the axis to be supported on the upstream and downstream support plates (11,12) and configured to introduce air from the first end side in the direction of the axis, wherein the plurality of premixing tubes (13;23) are disposed in rows shaped as a plurality of circles whose radial dimensions centered on the axis are different from each other, the neighboring premixing tubes (13;23) disposed in the same row are disposed equidistantly from each other in a circumferential direction, and portions of the premixing tubes (13;23) which are located in the plenum are formed with fuel introduction holes penetrating the premixing tubes (13;23) from outside to inside such that the fuel gas supplied from the plenum into the premixing tubes (13;23) via the fuel introduction holes in operation is mixed with the air in the premixing tubes (13;23) and is injected from second end sides of the premixing tubes (13;23) in the direction of the axis, characterized in that a length of the plenum in each row in the direction of the axis is set such that a flow velocity (v) of the fuel gas circulating in circumferential spaces between the plurality of premixing tubes (13;23) in a radial direction is constant.