To obtain a bulge processing apparatus capable of protecting buckling and also facilitating formation of a die.

A tube material 6 is inserted into a cavity 4 arranged for bellows formation by a die 1. Axial compression force is given to the tube material 6 by driving right and left pushing dies 8 and 16 by means of cylinders 10 and 23. Then, highly pressurized liquid is supplied into the tube material 6. An insertion hole 26 slidably fitting on an outer periphery of the tube material 6 is formed, and a moving member 34, in which a pressure hole 42 capable of inserting the right pushing die 16 and also communicating with the insertion hole 26 is formed, is also slidably supported to an axial direction of the tube material 6. Further, so that highly pressurized liquid supplied to the pressure hole 42 pressurizes the outer periphery of the tube material 6 to prevent buckling. The tube material 5 is processed while moving the inner shaft 22 so that an inner shaft 22 slidably inserted into the tube material 6 and a cylinder 33 moving the inner shaft 22 are provided.