  We show that the hybrid inflation is naturally realized in the framework of a
supersymmetric axion model, which is consistent with the WMAP observation if
the Peccei-Quinn symmetry breaking scale is around 10^{15}GeV. By solving the
post inflationary scalar dynamics, it is found that the scalar partner of the
axion, saxion, oscillates with large amplitude and its decay produces a huge
entropy and dilutes the axion. As a result, the axion coherent oscillation can
be the dominant component of the dark matter in the Universe. Cosmological
gravitino and axino problems are solved.
