There is provided a method by which a member having an undercut in the inner peripheral portion thereof can be formed readily. To form a material (1) formed with a recess (2) and the undercut (3) by using a swaging device, while the material (1) is first gripped by a clamper (11), and a mandrel (12) is inserted into the recess (2) of the material (1). The mandrel (12) used has an outside diameter equal to the inside diameter of a blind hole of an aimed product (fuel injection nozzle). Then, the material (1) is pushed in by the mandrel (12) to a position at which the material (1) abuts on a stopper (13), and the outside surface of the material (1) is struck by swaging dies (8) to perform a swaging operation. This swaging operation reduces the inside diameter of the recess (2) to the outside diameter of the mandrel (12), with the undercut (3) left.