  We discuss the model of the primordial black holes formation at the reheating
stage. These small massive black holes appear due to specific properties of the
compact extra dimensions. The latter gives rise the low energy model containing
the effective scalar field potential capable for the domain walls production.
Formed during inflation, these walls are quite dense so they collapse soon
after inflation ends. The discussion is performed within the scope of
multidimensional $f(R)$-gravity.
