To provide a through-duct freely length-adjustable corresponding to a hollow space of a through-hole portion of a hollow wall and for being mounted in the hollow wall in an air-tight manner.

The through-duct is integrally formed of a flexible and elastic synthetic resin in a cylindrical shape as a whole. On the outer periphery of a pipe material 1 at the side of an inner wall material 24, an external flange 7 on the end side and an internal flange 5 on the center side are provided between which an outer peripheral groove 8 is formed. On the outer periphery thereof at the side of an outer wall material 25, an internal flange 6 is provided at a site longer distant from the internal flange 5 than the hollow space. On the side of an end 1b further beyond the internal flange 6, an insertion guide portion 9 is formed longer than the hollow space. In an area of the outer peripheral groove 8, the outer periphery diameter of the pipe material 1 is almost equal to that of the through-hole of the inner wall material 24. At the insertion guide portion 9, it is almost equal to that of the through-hole of an outer wall material 25. One side 3 ranging from an approximately central site between both internal flanges 5, 6 is formed thicker with its diameter being smaller, and the other side 4 is formed thinner with its diameter being larger to be movable forward and backward over the outer periphery on the small diameter side 3.