To provide an external wall joint structure in which designability is not deteriorated even if the width size of the external wall joint is not constant and neither of the leading edges of the outermost side fin of a gasket are caught in mutually parallel counter face parts sandwiching the joint internal space of an external wall during construction.

A water cut-off gasket 3.. is fitted into an external wall joint 2.. the width size of which is not constant, in such a manner that the fitting-in depth position of the width-direction intermediate portion of an outermost side fin 4 facing the outdoor side becomes the same depth position between mutually parallel counter face parts 1a, 1a sandwiching a joint internal space. The outermost side fin 4 of the gasket 3 is of a round shape projected to the outside from the central part toward both leading edges 4a, 4a, which are warped to the outside and not along outer wall surfaces 1b, 1b in the portion where the both leading edges 4a, 4a project to the outdoor side further than the mutually parallel counter face parts 1a, 1a sandwiching the joint internal space in the fitted state.