  String cosmology is revisited from cosmological viewpoint of holographic
principle put forward by `t Hooft, and by Fischler and Susskind. It is shown
that the holography requires existence of a `graceful exit' mechanism, which
renders the Universe nonsingular by connecting pre- and post-big bang phases
smoothly. It is proven that flat Universe is consistent with the holography
only if it starts with an absolutely cold and vacuous state. Particle entropy
produced during the `graceful exit' apparently saturates the holography bound.
Open Universe can always satisfy the holography no matter what initial state of
the Universe is.
