PURPOSE: To conduct bulging of various kinds of a tapered tube at low cost by slitting a whole die to plural number of die elements in the longitudinal direction and combining these plural die elements corresponding to a forming size of the tapered tube.

CONSTITUTION: A forming die 12 is formed by combining upper/lower each plural die elements 11 selected corresponding to a diameter (d), length L of a tapered tube 1 and a metal tube stock is arranged in the forming die 12. Next, by applying a high pressure fluid body P on the inner diameter part of a metal tube stock in the forming die 12 for a prescribed time and by applying axial force to the end face of the metal tube stock with a pressure rod 15, bulging is executed. The forming die 12 has the max. diameter (d) at the center part in the axial direction and a tapered inner face reducing its diameter from the center part to both ends. As a result, a semiproduct having a length of 2L is obtained by bulging with the forming die. This semiproduct is cut at its center, two tapered tubes having a diameter of (d), length of L are obtained.