  The Starobinsky model predicts a primordial inflation period without the
presence of an inflaton field. The modified version of this model predicts a
simple time dependence for the Hubble parameter $H(t)$, which decreases slowly
between the Planck epoch and the end of the inflation, $H(t)=M_{\rm Pl}-\beta
M_{\rm Pl}^2 t$, where $\beta$ is a dimensionless constant to be adjusted from
observations. We investigate an inflaton model which has the same time
dependence for $H(t)$. A reverse engineered inflaton potential for the time
dependence of $H$ is derived. Normalization of the derived inflaton potential
is determined by the condition that the observed density fluctuations,
$\delta\rho/\rho\approx 10^{-5}$, are created at $\sim 60 e$-folds before the
end of inflation. The derived potential indicates an energy (mass) scale,
$M_{\rm end}\sim 10^{13} {\rm GeV}$, at the end of inflation. Using the slow
roll parameters, which are obtained from this potential, we calculate the
spectral index for the scalar modes $n_S$ and the relative amplitude of the
tensor to scalar modes $r$. A tensor contribution, $r\simeq 0.13$, and an
approximately Harrison-Zeldovich density perturbation spectrum, $n_S \simeq
0.95$, are predicted.
