This invention provides a forming method capable of forming tapered through holes at once with a single pressing operation. A hole is formed in a work with a piercing punch 1 mounted on a press by using a small-diameter punching portion 1a formed on the distal end of the punch 1. A hole 11 formed at this time has a section with an open lower end portion by pressing. By using this hole 11 as a guide, a tapering portion 1b having a diameter slightly larger than the diameter of this hole and a tapered surface T forms a tapered surface on the inner surface of the hole 11 in accordance with plastic working. A "burr 3a" is formed on the edge of the hole 11 by this plastic working. Formation of this "burr 3a" is prevented with a pressure portion 1c formed on the piercing punch 1. When the above process is performed in a single pressing operation, a plurality of tapered through holes can be formed easily.