An upsetting method high in productivity of an upsetting manufactured product is provided. An extruder 11 and a closed-die 20 divided into two pieces are prepared. The divided members 21 and 21 of the closed-die are combined with each other so that the tip end portion of an extrusion nozzle 13 is arranged in a cavity 25 of the closed-die 20 with the extruded raw material 1 extruded from the extrusion opening 15 of the extrusion nozzle 13 of the extruder 11. With this, the closed-die 20 is formed, and the extruded raw material 1 is clamped by and between clamping portions 22 and 22 provided at both the divided members 21 and 21. Subsequently, while extruding the extruded raw material 1 from the extrusion opening 15 of the extrusion nozzle 13 by the extruder 11, both the divided members 21 and 21 are moved in the forward direction with respect to the extrusion direction of the extruded raw material 1 at a speed slower than the extrusion speed in a state in which the extruded raw material 1 is clamped by and between the clamping portions 22 and 22 of both the divided members 21 and 21. Thereby, the exposed portion 2 of the extruded raw material 1 exposed between the extrusion opening 15 of the extrusion nozzle 13 and the clamping portions 22 of both the divided members 21 and 21 is expanded in diameter in the cavity 25.