  The previously introduced class of two-parametric phenomenological
inflationary models in General Relativity in which the slow-roll assumption is
replaced by the more general, constant-roll condition is generalized to the
case of $f(R)$ gravity. A simple constant-roll condition is defined in the
original Jordan frame, and exact expressions for a scalaron potential in the
Einstein frame, for a function $f(R)$ (in the parametric form) and for
inflationary dynamics are obtained. The region of the model parameters
permitted by the latest observational constraints on the scalar spectral index
and the tensor-to-scalar ratio of primordial metric perturbations generated
during inflation is determined.
