  We reconsider the matrix model formulation of type IIB superstring theory in
(9+1)-dimensional space-time. Unlike the previous proposal in which the Wick
rotation was used to make the model well-defined, we regularize the Lorentzian
model by introducing infrared cutoffs in both the spatial and temporal
directions. Monte Carlo studies reveal that the two cutoffs can be removed in
the large-N limit and that the theory thus obtained has no parameters other
than one scale parameter. Moreover, we find that three out of nine spatial
directions start to expand at some "critical time", after which the space has
SO(3) symmetry instead of SO(9).
