  In a series of papers, we pointed out that an anomalous $U(1)_A$ gauge
symmetry naturally solves various problems in grand unified theories (GUTs) and
that a horizontal gauge symmetry, $SU(2)_H$ or $SU(3)_H$, not only realizes the
unification of three generation quarks and leptons in fewer multiplets but also
solves the supersymmetric flavor problem. In this paper, we examine the
possibility that the Higgs sectors of the GUT symmetry and of the horizontal
symmetry are unified, that is, there are some Higgs fields whose vacuum
expectation values (VEVs) break both the GUT gauge symmetry and the horizontal
symmetry at the same time. Although the scale of the VEVs become too large to
suppress the flavor changing neutral current processes sufficiently, the
unification is possible. In addition, for the $SU(3)_H$ models, the $SU(3)_H$
gauge anomaly is cancelled in the unified models without introducing additional
fields in contrast with the previous models in which the Higgs sectors are not
unified.
