of the preliminary molded member 10 next the first tubular member 1 was prepared by welding a stainless pipe which had an internal diameter of 40 mm u03c6 an external diameter of 48 mm u03c6 and a length of 30 mm to a stainless pipe which had an internal diameter of 26 mm u03c6 an external diameter of 34 mm u03c6 and a length of 35 mm through a disc of stainless steel which had a thickness of 5 mm and an external diameter of 48 mm u03c6 and which had a center hole of diameter 26 mm u03c6 the second tubular member 2 was a pipe of stainless steel which had an internal diameter of 26 mm u03c6 an external diameter of 34 mm u03c6 and a length of 70 mm and which had one end formed with a chamfered portion 2b on its outer circumference in the mold the holding member 14 and the supporting member 16 were enclosed in the split wall 5 which was assembled by the mold flask 6 the side pressure member the pressure member the holding member 18 and the pressure member 9 were not assembled but were heated to 300 u00b0 c both the first tubular member 1 and.