  Primordial black holes (PBHs) with mass $10^{16}-10^{17}$ g almost escape
constraints from observations so could essentially contribute to dark matter
density. Hawking evaporation of such PBHs produces with a steady rate $\gamma$-
and $e^{\pm}$-radiations in MeV energy range, which can be absorbed by ordinary
matter. Simplified estimates show that a small fraction of evaporated energy
had to be absorbed by baryonic matter what can turn out to be enough to heat
the matter so it is fully ionized at the redshift $z\sim 5... 10$. The result
is found to be close to a borderline case where the effect appears, what makes
it sensitive to the approximation used. In our approximation, degree of gas
ionization reaches 50-100% by $z\sim 5$ for PBH mass $(3...7)\times 10^{16}$ g
with their abundance corresponding to the upper limit.
