To provide a sound proof joint and a method of its manufacture which provides a comfortable living space by having a sufficient impact absorbing effect and excellent soundproof performance.

On the inner face of an outer layer made of a synthetic resin, an inner layer is laminated made of a soft resin and having a maximum value of a buffer factor of 20 or less under a static stress of 0.1 to 1 MPa. The inner layer is made of an independent foaming body with an expansion ratio of 1.1 or more and a thickness of 2 mm or more in which a soft resin with rubber hardness in the non-foamed state is 50 or less is made to foam. The outer layer molded in advance is attached in a die for injection molding, and a soft resin impregnated with a carbonic acid gas is injected into the inner face of the outer layer within a cavity formed by the outer layer and the die for injection molding.