PURPOSE: To prevent the leakage of pressure liquid and damage of pipe during expansion of pipe and also to increase the number of times of use, by sloping the bottom face of groove for O-ring provided on the outer face of tip end of moving direction and constructing in such a way that the moving end part contacts with the O-ring by face.

CONSTITUTION: The groove 11 held O-ring at the end part 6 of pipe expander, is extended and the bottom face is sloped in the O-ring moving direction and then the moving end parts 11a, 12b are constructed so as to be contacted with the O-ring by face. When pressure liquid is introduced in the gap 10 of the pipe 3 between the O-ring 9-9 and the tip end part of the pipe expander 6, the above part of the pipe 3 is expanded and is press-bonded to the pipe plate 1 by pressure after inserting and setting the liquid pressure pipe expander in the pipe 3. At this time, the ring 9 is moved outside and contact pressure against the pipe 3 is increased accompanied by the movement because the bottom face is possessed inclination and then, leakage of the pressure liquid is prevented by complete sealing. Also, the tip part of the groove 11a in the groove 11 is made in contact state with the ring 9 by face because the curvature of the part 11 is made nearly the same to the sectional radius of the ring 9 and the stress given to the part 11a, is dispersed and then, the corner part 11 is not broken down, also.