To provide a packing mounting structure which can reliably seal a gap even when a gap between a sheath pipe and a pipe inserted to the interior of the sheath pipe is large or small.

In a packing mounting structure for sealing a gap between a sheath pipe 1 which is buried so as to penetrate a base B of a building and a pipe 2 inserted into the sheath pipe 1 with a packing 4 on the edge part of the sheath pipe 1, a top end part outer circumferential surface of a socket member 3 to which the packing 4 is externally inserted is formed to be a tapered surface 3a, and an inner circumferential surface of the packing 4 is configured so as to be a tapered surface 4a corresponding to the shape of the top end part outer circumferential surface of the socket member 3. By adopting such a configuration, even when a gap between the sheath pipe 1 and the socket member 3 is large or small, tapered surfaces 4a, 4b of the packing 4, the tapered surface 3a of the socket member 3 and the inner circumferential surface of the sheath pipe 1 are completely adhered, the gap can be reliably sealed and, therefore, the invasion of termite or the like can be completely blocked.