  We derive a long distance effective action for space-time coordinates from a
IIB matrix model. It provides us an effective tool to study the structures of
space-time. We prove the finiteness of the theory for finite $N$ to all orders
of the perturbation theory. Space-time is shown to be inseparable and its
dimensionality is dynamically determined. The IIB matrix model contains a
mechanism to ensure the vanishing cosmological constant which does not rely on
the manifest supersymmetry. We discuss possible mechanisms to obtain realistic
dimensionality and gauge groups from the IIB matrix model.
