To facilitate air-tightness maintenance, to hardly generate the explosion of air expansion tube and to improve feelings in wearing by inserting, connecting the other end of air expansion tube, which closes one end, to the branch tube of each T-shaped connecting tube and binding an auxiliary expansion tube through a connecting band to the other end of that air expansion tube while overlapping it.

The other end of air expansion tube 30, which closes one end 31 and opens the other end, is inserted and linked to the branch tube of each T-shaped connecting tube and the state of overlapping an auxiliary expansion tube 32 is provided. This auxiliary expansion tube 32 is connected at a connecting part with the T-shaped tube by a connecting band 11 while having a function for reinforcing strength and reducing the expansion of the air expansion tube 30. Then, a step 6 expanding an outer diameter is formed at the branch tube top end of the T-shaped connecting tube, the inner diameter of the expansion tube 30 is engaged with the reduced step and when expanding the expansion tube 30, power for the expansion tube 30 to be dropped is received so that its drop can be prevented. When air is injected into an air injection tube, the T-shaped tube and expansion tube 30 try to expand but the connecting band 11 suppresses this expansion and keeps the air tightness of the connecting tube.