  In this work we investigate a inflationary scenario generated by a large
scalar field $\phi$ that non-minimally couples to a $f(R)$ modified gravity
model. For a Starobinsky's like model, it is found that along a particular flat
direction, the scalar potential takes a simple form $V = (M_p^4/4)
[V(\phi)/\alpha(\phi)^2]$ where $\alpha(\phi)$ is a non-minimal coupling to
Ricci scalar $R$ in the model. The inflation, therefore, is effectively
represented as a single field inflaton scenario. For a specific example, such
as a scalar potential $V(\phi) = \mu_1\phi^2 + \mu_2\phi^4$, we found that the
predictions match nicely in the $1\sigma$ confidence level of \textit{Plank}
TT, TE, EE+lowP combination data of Planck 2015 CMB data for $ 0<\mu_3 \leq
100$, where $\mu_3 := |\mu_1|/(\mu_2 M_p^2)$. For example, taking $\mu_3 =
0.01$ the scalar-to-tensor ratio $r=0.0004$ and and spectral index $n_s =
0.96985$ for $N_\ast = 50$ while taking $\mu_3 = 100.0$ produces $r= 0.03$ and
$n_s = 0.96359$ for $N_\ast = 60$.
