  We give a general way of representing the crystal (base) corresponding to the
intgrable highest weight modules of quantum Kac-Moody algebras, which is called
polyhedral realizations. This is applied to describe explicitly the crystal
bases of integrable highest weight modules for arbitrary rank 2 Kac-Moody
algebra cases, the classical A_n-case and the affine A^{(1)}_{n-1}-case.
