To provide a core for hydrostatically bulging a tubular material and a forming method of a hollow member using the same by which the hollow member having the amount of bulging not smaller than that of conventional methods is obtained even when forming a tube stock by the amount of axial push smaller than that of the conventional methods in the hydrostatic bulging of the tubular material.

In the hydrostatic bulging in which the end faces of the tube stock are axially pushed while loading the inside surface of the tube stock with fluid pressure, the thickness distribution is approximately equalized by inserting the cores 21 for hydrostatic bulging into the tube stock 10 and forming the inside surface of the hollow member 14 by bringing the cores 21 into contact with the inside surface of the tube stock 10 when the tube stock 10 is deformed. The cores 21 for hydrostatic bulging have a flow passage 23 for introducing the fluid from the outside of the tube stock 10 when loading the inside surface of the tube stock 10 with the fluid pressure. Further, cores 24, 25 for hydrostatic bulging can be taken into and out of the inside of the tube stock 10 from both end parts of the tube stock 10 by making them dividable.