PURPOSE: To enable the mandrel to be inserted even if the hole diameter of the pipe inlet owing to seal welding becomes smaller by fitting the sealing members that can expand the diameter to both end parts of the mandrel, respectively.

CONSTITUTION: When working fluid is introduced through a high pressure hose 8, it imparts pressure to an O-ring 6 while pressing the first piston valve 11. This causes a backup ring 3 to be force expanded in the diametral direction to close the spacing from a heat transfer pipe 14. The first piston valve 11 closes a radial channel 5. The pressure of the working fluid further passes through the radial channel 5' to force expand an elastic barrel 7. As a result, the backup ring 3' or O-ring 2' is force expanded, by which the spacing from the heat transfer pipe 14 is closed. Thereafter, the second piston valve 12 opens a communicating path 15, allowing the working fluid to flow into the space S between the mandrel 1 and heat transfer pipe 14. This working fluid acts upon the entire surface of the heat transfer pipe, thereby force expanding the same.