  We study the possibility of family unification on the basis of SO(2N) gauge
theory on the five-dimensional space-time, $M^4\times S^1/Z_2$. Several SO(10),
$SU(4) \times SU(2)_L \times SU(2)_R$ or SU(5) multiplets come from a single
bulk multiplet of SO(2N) after the orbifold breaking. Other multiplets
including brane fields are necessary to compose three families of quarks and
leptons.
