  We show that a very flat potential of inflaton required for a sufficient
inflation is naturally obtained in $N=1$ supergravity by imposing a discrete
$R$-invariance $Z_{n}$. Several cosmological constraints on parameters in the
inflaton superpotential are derived. The reheating temperature turns out to be
$(1-10^8)$GeV for the cases of $n$=3--10. Baryogenesis in this model is also
discussed briefly.
