  We present a scenario where a $Z_2$-symmetric scalar field $\phi$ first
drives cosmic inflation, then reheats the Universe but remains
out-of-equilibrium itself, and finally comprises the observed dark matter
abundance, produced by particle decays \`{a} la freeze-in mechanism. We work
model-independently without specifying the interactions of the scalar field
besides its self-interaction coupling, $\lambda\phi^4$, non-minimal coupling to
gravity, $\xi\phi^2R$, and coupling to another scalar field, $g\phi^2\sigma^2$.
We find the scalar field $\phi$ serves both as the inflaton and a dark matter
candidate if $10^{-9}\lesssim \lambda\lesssim g\lesssim 10^{-7}$ and $3
\rm{keV} \lesssim m_{\rm \phi}\lesssim 85 \rm{MeV}$ for $\xi=\mathcal{O}(1)$.
Such a small value of the non-minimal coupling is also found to be of the right
magnitude to produce the observed curvature perturbation amplitude within the
scenario. We also discuss how the model may be distinguished from other
inflationary models of the same type by the next generation CMB satellites.
