  We consider the simple inflationary model with peculiarity in the form of
"plateau" in the inflaton potential. We use the formalism of coarse-grained
field to describe the production of metric perturbations $h$ of an arbitrary
amplitude and obtain non-Gaussian probability function for such metric
perturbations. We associate the spatial regions having large perturbations
$h\sim 1$ with the regions going to primordial black holes after inflation. We
show that in our model the non-linear effects can lead to overproduction of the
primordial black holes.
