An exhaust manifold assembly including at least two manifold part members, an annular end projection of one manifold part member receiving an annular end projection of the other manifold part member to form a connection portion, further including (a) an annular slidable tubular member made of a metal material having good heat resistance and oxidation resistance and inserted into a gap between the annular end projections of both manifold part members; and (b) a flexible pipe having a corrugated pipe portion and tubular portions integrally extending from both ends of the corrugated pipe portion, the tubular portions of the flexible pipe being continuously welded to outer surfaces of the manifold part members, whereby the connection portion is completely sealed by the flexible pipe.