  We present an explicit cosmological model where inflation and dark energy
both could arise from the dynamics of the same scalar field. We present our
discussion in the framework where the inflaton field $\phi$ attains a nearly
constant velocity $m_P^{-1} |d\phi/dN|\equiv \alpha+\beta \exp(\beta N)$ (where
$N\equiv \ln a$ is the e-folding time) during inflation. We show that the model
with $|\alpha|<0.25$ and $\beta<0$ can easily satisfy inflationary constraints,
including the spectral index of scalar fluctuations ($n_s=0.96\pm 0.013$),
tensor-to-scalar ratio ($r<0.28$) and also the bound imposed on $\Omega_\phi$
during the nucleosynthesis epoch ($\Omega_\phi (1 {\rm MeV})<0.1$). In our
construction, the scalar field potential always scales proportionally to the
square of the Hubble expansion rate. One may thereby account for the two vastly
different energy scales associated with the Hubble parameters at early and late
epochs. The inflaton energy could also produce an observationally significant
effective dark energy at a late epoch without violating local gravity tests.
