A spherical annular seal member (58) includes: a spherical annular base member (56) defined by a cylindrical inner surface (52), a partially convex spherical surface (56), and end faces (54, 55); and an outer layer (57) formed integrally with the partially convex spherical surface (53) of the spherical annular base member (56). The spherical annular base member (56) includes a reinforcing member (5) made from a compressed metal wire net (4) and a heat-resistant material filling meshes of the metal wire net (4) of the reinforcing member (5), integrated with the reinforcing member (5) in mixed form, and containing expanded graphite and an organic phosphorus compound. The outer layer (56) includes a heat-resistant material constituted by a heat-resistant sheet member (6) containing expanded graphite and an organic phosphorus compound, and a reinforcing member constituted by a metal wire net (4) integrated with the heat-resistant material in mixed formed.