Let M be a Dupin hypersurface in the unit sphere S n+1 with six distinct principal curvatures. We will prove in the present paper that M is either diffeomorphic to SU (2) × SU (2)/Q 8 or homeomorphic to a tube around an embedded 5-dimensional complex Fermat hypersurface X 5 (2) in S 13 , where Q 8 ⊂ SU (2) = Sp(1) denotes the subgroup {±1, ±i, ±j, ±k} and X 5 (2) = {[z 0 , z 1 , · · · z 6 ] ∈ CP 6 |z 2 0 + z 2 1 + · · ·+ z 2 6 = 0}. Moreover, in the former case, all of the focal manifolds are diffeomorphic to S 3 × RP 2 ; In the latter case, one of the focal manifolds is homeomorphic to X 5 (2).