PURPOSE: To reduce a manufacturing cost by separately forming a cylindrical shell net intermediate part, arranging the dies of fixed and movable types as well and flaring the shell net intermediate part in an elliptic conical shape via the movement of the movable die.

CONSTITUTION: A fixed die 2 is provided on a die base 4 and a pair of movable dies 3 slidable in the major axial direction coming in contact with the die 2 are respectively arranged. Separately a cylindrical shell net intermediate part 15 is formed in advance by welding by superposing both ends of the shell net stock. A shell net 16 is formed by pushing down the shell net intermediate part 15 to the base 4 by a press 12 along the external shape face of the fixed die 2 and moving the movable die 3 to the outside via a piston rod 11 until its attaining all elliptic conical shape. A product is then obtd. by welding reinforcing frame, reinforcing line and bottom plate. Welding places are thus remarkably reduced and the material yield is also improved. Consequently the manufacturing cost is reduced.