  We consider a superradiance effect around rotating dilatonic black holes. We
analyze two cases: one is an exact solution with the coupling constant
$\alpha=\sqrt{3}$, which effective action is reduced from the 5-dimensional
Kaluza-Klein theory, and the other is a slowly rotating dilatonic black holes
with arbitrary coupling constant. We find that there exists a critical value
($\alpha \sim 1$), which is predicted from a superstring model, and the
superradiant emission rate with coupling larger than the critical value becomes
much higher than the Kerr-Newman case ($\alpha=0$) in the maximally charged
limit. Consequently, 4-dimensional primordial black holes in higher dimensional
unified theories are either rotating but almost neutral or charged but
effectively non-rotating.
