  We study the spacetime structures which are described by the IIB matrix model
with orientifolding. Matrix orientifolding that preserves supersymmetries
yields the mirror image point with respect to a four-dimensional plane for each
spacetime point that corresponds to the eigenvalue of the bosonic matrix. In
order to consider the upper bound on the distance between two eigenvalues in
this model, we calculate the effective action for the eigenvalues up to
two-loop. The eigenvalues distribute in a tubular region around the
four-dimensional plane.
