Abstract: A die assembly comprising a reinforcement case formed with a tapered bore, a forming die inserted into the tapered bore from therebelow, and a stock die supporting the forming die in fitting contact with the bottom of the forming die and formed at its lower portion with a guide stem of reduced diameter. The guide stem extends downward through the center of a threaded bore formed in the lower portion of the reinforcement case. A pushing-up member having a center bore through which the guide stem extends freely movably is screwed in the threaded bore of the reinforcement case and supports the stock die. The forming die and the stock die are formed with knockout pin passageways in alignment with each other.