PURPOSE: To form a can drum having good finished surface without giving material fatigue to the can material by using sectional piecelike parts that expand and contract with movement of vertically movable axis for internal shape forming, in a can drum forming device that uses a forming inner die and outer die.

CONSTITUTION: In forming a convex bulged part on a can drum except upper and lower ends, a can drum (a) is formed by a forming outer die 1 and an inner die 6. A forming die face 2 is provided on the forming outer die 1 and its upper end is fixed by an upper part supporting plate 3. A receiving port 4 for the can drum (a) is opened at the lower end and closed with a movable board 5 that can be opened and closed freely. Plural sectional-piecelike segments 7 formed to a die face pressing part by putting a space filling material 12 in adjoining spaces are used for a forming inner die 6. A rod 17 that moves vertically is inserted into its central hole 16, and a tapered part of the forming inner die 6 that can expand and contract is provided on the fitting face of the rod 17 and central hole 16. Thus, bulging work excellent in surface condition can be performed on the can drum without giving material fatigue to the can material.