  We investigate the constraint on the abundance of primordial black holes
(PBHs) and the spectral index $n$ of primeval density perturbations given by
the ionizing photon background at the epoch of reionization. Within the
standard inflationary cosmogony, we show that the spectral index $n$ of the
power-law power spectrum of primeval density perturbations should be $n<$1.27.
Since the universe is still optical thick at the reionization redshift $z\sim
6$ - 8, this constraint is independent of the unknown parameter of reheating
temperature of the inflation. The ionizing photon background from the PBHs can
be well approximated by a power law spectrum $J(\nu)\propto{\nu}^3$, which is
greatly different from those given by models of massive stars and quasars.
