To continuously execute a pipe expanding work in high efficiency by using a pressurizing means providing a movable piston, acting simultaneously thrust applied to the movable piston to a pipe stock end part with fluid for internal pressure.

A pipe stock 4 having an outside diameter smaller than the min. diameter of a hollow part 10 is inserted into a metallic mold 3 having the hollow part 10 reduced in the diameter toward the end parts in the longitudinal direction. A fixed cylinder 1 providing the movable piston 2 is arranged as the pressurizing means at the outside of the end part of the metallic mold 3. The movable piston 2 has a through hole 8 having smaller cross section than a pressure receiving area 9 so as to be actuated by utilizing the fluid 11 for internal pressure and water from a pressurized water supplying means flows into the pipe stock 4 through the through hole 8 and the thrust is simultaneously exerted on the movable piston 2. By this method, the pressurizing means is integrated and the pipe expanding work is continuously executed and the efficiency can be improved.