PURPOSE: To easily from a tapered section in cylindrical thin can bodies fitted into sectional dies, by expanding sectional dies composed of plural slender pieces of sectional dies and having a polygonal or a cylindrical cross section to a form of funnel.

CONSTITUTION: A tool A is formed of sectional dies 2 composed of plural slender pieces of sectional dies 2a and having a regular polygonal or a cylindrical cross section, and a central shaft 5 which is inserted into a central hole 2h of the dies 2 and having a tapered section. A cylindrical thin can body 1 is fitted into the tool A. Then, the above mentioned central shaft 5 is lowered and the pieces of sectional dies 2a are expanded to form a funnel in such a way that the diameter of the funnel at the bottom 1a of the can body is maintained as it is and the diameter is increased as going toward the upper end 1b of the can body. The can body 1 is formed to a tapered thin can body 1'. As the central shaft 5 ascends, a clearance is produced between the outer peripheral surface 2a1 of the pieces of sectional dies 2a and the can body 1', and the can body 1' can be taken out easily from the sectional dies 2.