  The spontaneous breakdown of SO(10) symmetry of the IIB matrix model has been
studied by using the improved mean field approximation (IMFA). In this report,
the eighth-order contribution to the improved perturbative series is obtained,
which involves evaluation of 20410 planar two-particle irreducible vacuum
diagrams. We consider SO(d)-preserving configurations as ansatz (d=4,7). The
development of plateau, the solution of self-consistency condition, is seen in
both ansatz. The large ratio of the space-time extent of d-dimensional part
against the remaining (10-d)-dimensional part is obtained for SO(4) ansatz
evaluated at the representative points of the plateau. It would be interpreted
as the emergence of four-dimensional space-time in the IIB matrix model.
