PURPOSE: To seal a flow passage and stabilize a weakened part without causing vibration by expanding an inner wall of a tube of a heat exchanger unstable due to the damage of a wall part by an expansion sleeve.

CONSTITUTION: When a stabilizer 7 is pressed in until it comes into contact with a machining face of tube sheet 2, a breakage point 23 is carried so that it is positioned between expansion sleeves 14 and 14a and is stabilized. The stabilizer 7 is inserted into one of two tubes until it reaches a flange. An upper expansion sleeve 14 is positioned just above the breakage point 23, and a second expansion sleeve 14a is considerably apart from the breakage point 23. A joint 9 having a sliding sleeve 10 is below the expansion sleeve 14a. When a rod 8 is rotated by a rotary tool 31, tongs 16, 16a are pressed against an inner wall of the tube through a cam. A lower end part of the rod 8 is roll-tightened to seal the tube 5 after clamping the expansion sleeve.