Recently, the Gaussian expansion method has been applied to investigate the dynamical generation of 4d space-time in the IIB matrix model, which is a conjectured nonperturbative definition of type IIB superstring theory in 10 dimensions. Evidence for such a phenomenon, which is associated with the spontaneous breaking of the SO(10) symmetry down to SO(4), has been obtained up to 7-th order calculations. Here we apply the same method to a simplified model, which is considered to exhibit an analogous spontaneous symmetry breaking via the same mechanism as conjectured for the IIB matrix model. The results up to 9-th order demonstrate a clear convergence, which allows us to unambiguously identify the actual symmetry breaking pattern by comparing the free energy of possible vacua and to calculate the extent of "space-time" in each direction. §1. IntroductionIt has long been believed that matrix models may be useful as a nonperturbative formulation of string theory, and hence play an important role similar to the lattice formulation of quantum field theory. Indeed, matrix models have been quite successful in formulating non-critical string theory, and after the development of such notions as string duality and D-branes, the idea has been extended also to critical strings. The IIB matrix model 1) is one such proposal, which is conjectured to be a nonperturbative definition of type IIB superstring theory in 10 dimensions. It is a supersymmetric matrix model, which can be formally obtained as the zero-volume limit of 10d SU (N ) super Yang-Mills theory.In this model space-time is represented by the eigenvalue distribution of ten bosonic matrices. 2) If the distribution collapses dynamically to a four-dimensional hypersurface, which, in particular, requires the SO(10) symmetry of the model to be spontaneously broken, we may naturally understand the dimensionality of our space-time as a result of the nonperturbative dynamics of superstring theory. In Ref.3) the first evidence for the above scenario was obtained by calculating the free * ) 488 J. Nishimura, T. Okubo and F. Sugino energy of space-times with various dimensionalities using the Gaussian expansion method up to 3rd order. Higher-order calculations, 4), 5) as well as tests of the method itself in simpler models, 6), 7) have strengthened the conclusion considerably. Further evidence for the emergence of a four-dimensional space-time based on perturbative calculations around fuzzy-sphere like solutions is provided in Ref. 8).While these results are certainly encouraging, it is desirable to understand the mechanism responsible for the spontaneous symmetry breaking (SSB) of rotational symmetry. In Ref. 9) it is pointed out that the phase of the fermion determinant favors lower-dimensional configurations, since the phase becomes stationary around such configurations. * ) Indeed, Monte Carlo simulations show that SSB does not occur in various models without such a phase factor. 12)-14) Unfortunately, including the effects of the phase in Monte Carlo simulations is techni...