  The 21 cm signal produced by non-evaporating primordial black holes (PBHs) is
investigated. X-ray photons emitted by accretion of matter onto a PBH ionize
and heat the intergalactic medium (IGM) gas near the PBH. Using a simple
analytic model, we show that this X-ray heating can produce an observable
differential 21 cm brightness temperature. The region of the observable 21 cm
brightness temperature can extend to 1-10 Mpc comoving distance from a PBH
depending on the PBH mass. The angular power spectrum of 21 cm fluctuations due
to PBHs is also calculated. The peak position of the angular spectrum depends
on PBH mass, while the amplitude is independent of PBH mass. Comparing this
angular power spectrum with the angular power spectrum caused by primordial
density fluctuations, it is found that both of them become comparable if
$\Omega_{{\rm PBH}} = 10^{-11} (M/10^{3} M_\odot)^{-0.2}$ at $z=30$ and
$10^{-12} (M/10^{3} M_\odot)^{-0.2}$ at $z=20$ for the PBH mass from $10
M_\odot $ to $10^8 M_\odot $. Finally we find that the Square Kilometer Array
can detect the signal due to PBHs up to $\Omega_{\rm PBH}=10^{-5} (M/10^{3}
M_\odot)^{-0.2}$ at $z=30$ and $10^{-7} (M/10^{3} M_\odot)^{-0.2}$ at $z=20$
for PBHs with mass from $10^2 M_\odot $ to $10^8 M_\odot $.
