  The type IIB matrix model has been investigated as a possible nonperturbative
formulation of superstring theory. In particular, it was found by Monte Carlo
simulation of the Lorentzian version that the 9-dimensional rotational symmetry
of the spatial matrices is broken spontaneously to the 3-dimensional one after
some "critical time". In this paper we develop a new simulation method based on
the effective theory for the submatrices corresponding to the late time. Using
this method, one can obtain the results for $N\times N$ matrices by simulating
matrices typically of the size $O(\sqrt{N})$. We confirm the validity of this
method and demonstrate its usefulness in simplified models.
