  The conventional supersymmetric grand unified theories suffer from two
serious problems, the large mass splitting between doublet and triplet Higgs
multiplets, and the too long lifetime of the proton. A unification model based
on a semi-simple group SU(5)_{GUT} \times U(3)_H has been proposed to solve
both of the problems simultaneously. Although the proposed model is perfectly
consistent with observations, there are various mysteries. In this paper, we
show that such mysterious features in the original model are naturally
explained by embedding the model into the brane world in a higher dimensional
space-time. In particular, the relatively small gauge coupling constant of the
SU(5)_{GUT} at the unification energy scale is a consequence of relatively
large volume of extra dimensions. Here, we put the SU(5)_{GUT} gauge multiplet
in a 6-dimensional bulk and assume all fields in the U(3)_H sector to reside on
a 3-dimensional brane located in the bulk. On the other hand, all chiral
multiplets of quarks, leptons and Higgs are assumed to reside on a 3-brane at a
T^2/Z_4 orbifold fixed point. The quasi-N=2 supersymmetry in the hypercolor
U(3)_H sector is understood as a low-energy remnant of the N=4 supersymmetry in
a 6-dimensional space-time. We further extend the 6-dimensional model to a
10-dimensional theory. Possible frameworks of string theories are also
investigated to accommodate the present brane-world model. We find that the
type IIB string theory with D3-D7 brane structure is an interesting candidate.
