A method of forming a rib having an overhanging part by cutting a workpiece made of an aluminium alloy, the method of forming a rib comprising: providing a T-cutter, which is provided with a shank and a head provided at an end of the shank, the head defining a plurality of bottom and top edge parts alternately arranged in a peripheral direction of the T-cutter, each of the bottom edge parts having a cutting edge formed by a ridgeline defined by intersection between a rake face of the bottom edge part and a bottom face, a side face, and an R-corner therebetween of the bottom edge part, each of the top edge parts having a cutting edge formed by a ridgeline defined by the intersection between a rake face of the top edge part and a top face, a side face, and an R-corner therebetween of the top edge part, wherein the T-cutter is produced by the steps of: producing a tool material by molding and sintering cemented carbide powder to have a side view in the form of substantially a T-shape close to the completed shape of the T-cutter; and grinding the tool material so as to produce a T-cutter having the shank, the head and the bottom and top edge parts (16, 18) integrally formed of a single material of the cemented carbide powder; the method further comprising the steps of: forming a rib part of said workpiece, mounted to a table of five-axis machine tool having three linear feed axes and two rotational feed axes, by using an end mill attached to the spindle, while leaving a thickness of at least a width of said overhanging part; attaching said produced T-cutter to a spindle of the five-axis machine tool rotating the spindle; and moving said workpiece mounted to the table of five-axis machine tool and said T-cutter relative to each other under five-axis control of the five-axis machine tool along the overhanging part in the form of a three-dimensional undercut shape defined by a curved surface which is slanted with respect to a plane vertical to the axial line of the T-cutter, after determining processing conditions so as to remove 100 cm 3 to 1000 cm 3 of material from the workpiece per minute. providing a T-cutter, which is provided with a shank and a head provided at an end of the shank, the head defining a plurality of bottom and top edge parts alternately arranged in a peripheral direction of the T-cutter, each of the bottom edge parts having a cutting edge formed by a ridgeline defined by intersection between a rake face of the bottom edge part and a bottom face, a side face, and an R-corner therebetween of the bottom edge part, each of the top edge parts having a cutting edge formed by a ridgeline defined by the intersection between a rake face of the top edge part and a top face, a side face, and an R-corner therebetween of the top edge part, wherein the T-cutter is produced by the steps of: producing a tool material by molding and sintering cemented carbide powder to have a side view in the form of substantially a T-shape close to the completed shape of the T-cutter; and grinding the tool material so as to produce a T-cutter having the shank, the head and the bottom and top edge parts (16, 18) integrally formed of a single material of the cemented carbide powder; producing a tool material by molding and sintering cemented carbide powder to have a side view in the form of substantially a T-shape close to the completed shape of the T-cutter; and grinding the tool material so as to produce a T-cutter having the shank, the head and the bottom and top edge parts (16, 18) integrally formed of a single material of the cemented carbide powder; the method further comprising the steps of: forming a rib part of said workpiece, mounted to a table of five-axis machine tool having three linear feed axes and two rotational feed axes, by using an end mill attached to the spindle, while leaving a thickness of at least a width of said overhanging part; attaching said produced T-cutter to a spindle of the five-axis machine tool rotating the spindle; and moving said workpiece mounted to the table of five-axis machine tool and said T-cutter relative to each other under five-axis control of the five-axis machine tool along the overhanging part in the form of a three-dimensional undercut shape defined by a curved surface which is slanted with respect to a plane vertical to the axial line of the T-cutter, after determining processing conditions so as to remove 100 cm 3 to 1000 cm 3 of material from the workpiece per minute. forming a rib part of said workpiece, mounted to a table of five-axis machine tool having three linear feed axes and two rotational feed axes, by using an end mill attached to the spindle, while leaving a thickness of at least a width of said overhanging part; attaching said produced T-cutter to a spindle of the five-axis machine tool rotating the spindle; and moving said workpiece mounted to the table of five-axis machine tool and said T-cutter relative to each other under five-axis control of the five-axis machine tool along the overhanging part in the form of a three-dimensional undercut shape defined by a curved surface which is slanted with respect to a plane vertical to the axial line of the T-cutter, after determining processing conditions so as to remove 100 cm 3 to 1000 cm 3 of material from the workpiece per minute.