  We study the possibility of complete family unification in higher-dimensional
space-time. Three families of matters in SU(5) grand unified theory are derived
from a single bulk multiplet of SU(N) gauge group (N >= 9) in the framework of
S^1/Z_2 orbifold models. In the case of the direct orbifold breaking down to
the standard model gauge group, there are models in which bulk fields from a
single multiplet and a few brane fields compose three families of quarks and
leptons.
