  It has not been clarified whether a matrix model can describe various vacua
of string theory. In this paper, we show that the IIB matrix model includes
type IIA string theory. In the naive large N limit of the IIB matrix model,
configurations consisting of simultaneously diagonalizable matrices form a
moduli space, although the unique vacuum would be determined by complicated
dynamics. This moduli space should correspond to a part of perturbatively
stable vacua of string theory. Actually, one point on the moduli space
represents type IIA string theory. Instead of integrating over the moduli space
in the path-integral, we can consider each of the simultaneously diagonalizable
configurations as a background and set the fluctuations of the diagonal
elements to zero. Such procedure is known as quenching in the context of the
large N reduced models. By quenching the diagonal elements of the matrices to
an appropriate configuration, we show that the quenched IIB matrix model is
equivalent to the two-dimensional large N N=8 super Yang-Mills theory on a
cylinder. This theory is nothing but matrix string theory and is known to be
equivalent to type IIA string theory. As a result, we find the manner to take
the large N limit in the IIB matrix model.
