  Starobinsky model, which has a Ricci scalar squared term $R^{2}$ in its
action, is one of the most promising inflation models from the viewpoint of
Cosmic Microwave Background observations. However, it is well known that
observational predictions of this model are quite sensitive to the existence of
$R^{m}$ $(2<m)$ terms, whose absence is just assumed. In this paper, we clarify
that the observational predictions of $D$-dimensional ($4<D$) extended
Starobinsky model are less sensitive to such terms than those of the original
4-dimensional model.This result make it easier to construct Starobinsky-like
models in higher dimensions.
