PURPOSE: To reinforce thin metal tubes being used as house gate and fence materials by lining inside surface of thin metal tubes such as aluminum, with steel plate as lining material tightly fixed therein.

CONSTITUTION: Steel plate 3 to be used as lining inside of the tube 1 of thin metal such as aluminum, is formed with such a shape as to fix the inside of the tube 1 and at the both edges thereof, a clearance 2 is provided. Because the clearance 2 acts as play to adjust the diameter or size of the tube 3, insertion and tight fixation against the inner wall of the tube 1 is ensured with ease by operating catch 4. Accordingly, fragility of the soft outer tube such as of thin decorated aluminum is perfectly compensated by the rigidity of the steel pipe when tube is constructed watertight.