  We investigate the influence of hadron injection from evaporating primordial
black holes (PBHs) in the early stage of the primordial nucleosynthesis era (t
= 10^{-3} - 10^4 sec). The emitted quark-antiquark pairs or gluons immediately
fragment into a lot of hadrons and scatter off the thermal plasma which is
constituted by photons, electrons and nucleons. For the relatively low mass
holes we point out that the dominant effect is the inter-conversion between
ambient proton and neutron through the strong interaction induced by the
emitted hadrons. Even after the freeze-out time of the week interactions
between neutron and proton, more neutrons are produced and the synthesized
light element abundances could be drastically changed. Comparing the
theoretical predictions with the observational data, we constrain the PBH's
density and their lifetime. We obtain the upper bound for PBH's initial mass
fraction, \beta < 10^{-20} for 10^8 g < M < 10^{10} g, and \beta < 10^{-22} for
10^{10} g < M < 3 * 10^{10} g.
