The residual finiteness of the free product of isomorphic free groups with a single finitely generated amalgamated subgroup will be shown. The method used is that of reference [Z], extended by use of theorem of Marshall Hall. The theorem generalizes a result of G. Baumslag [3], on the free product of free groups with a cyclic amalgamated subgroup. A few lemmas will be needed in the proof of the theorem. The proof of the first lemma appears here, while the others appear in reference [I].