To impart a pressure required for a whole tube inner face of a working region in a process of working, to prevent a wrinkle and a shape defect in bending/flattening a tube, to commonly use for manufacturing worked products having various cross sectional sizes and to quickly take in/out a core.

The core 10 is provided with a cylindrical or bag like hollow body 1 made of an elastic material and a fluid supply means to supply/discharge a fluid for bulging a peripheral length of the hollow body into/from its inside space. The core is arranged in the tube of plastic deforming section of a tube stock of a metal tube, a fluid is supplied in the inside space with a pressure not causing plastic deformation of the metal tube, the hollow body made of an elastic material is bulged, its outer peripheral face is brought into tight contact with an inner peripheral face of the metal tube, thereafter, a prescribed working is conducted.