  We hypothesize that the vacuum is spontaneously shifted from the self-dual
point in the moduli space, at which all dimensions are compact, and that the
true vacuum results from the decompactification of the four space-time
dimensions and remains in the vicinity of the self-dual point for extra
dimensions. On the basis of our hypotheses we explore new types of the coupling
unification which are not realized in the four-dimensional effective theory but
in the framework of the higher dimensional theory. In the $SU(6) \times
SU(2)_R$ string model, if the SU(6) gauge group lives in the world-volume of
9-branes and the $SU(2)_R$ in the world-volume of 5-branes, or {\it vice
versa}, unification solutions can be found in the vicinity of the self-dual
point.
