In this paper we estimate the
homology torsion module of an infinite cyclic covering space of an n-manifold by the
homology of a Poincaré duality space of dimension n− 1. To be concrete, we apply
it to knot theory. In particular, it follows that any ribbon n-knot K ⊂ Sn+2(n ≧ 3)
is unknotted if π1(Sn+2− K)≅Z. We add also in this paper a somewhat geometric
proof to this unknotting criterion.