  We consider the influence of decaying dark matter (DM) particles on the
characteristics of 21 cm absorption in spectra of distant radio-loud sources -
"21 cm forest" - from minihaloes with masses $M=10^5-10^7\msun$ virialized at
$z_{vir} = 10$. We use 1D self-consistent hydrodynamic description to study
evolution of minihaloes, and follow up their absorption characteristics from
turnaround to virialization. We find that in the presence of decaying dark
matter both thermal and dynamical evolution of minihaloes demonstrate
significant deviation from those in the model without dark matter decay
(standard recombination). We show that optical depth in 21 cm line is strongly
suppressed in the presence of decaying particles: for $M=10^5-10^6\msun$
decaying dark matter with the energy rate deposited in baryonic gas $\xi_{L} =
0.59\times 10^{-25}$ s$^{-1}$ - the current upper limit of the energy deposit -
decreases the optical depth and the equivalent width by an order of magnitude
compared to the standard recombination. Thus additional ionization and heating
from decaying DM particles almost "erases" absorption features from minihaloes
with $M=10^5-10^6\msun$ for $\xi \simgt 0.3\xi_L$, which consequently
considerably decreases the number of strong absorptions: for example, the
number of absorptions with the equivalent width $W_\nu^{obs} \simgt 0.3$ kHz at
$z\simeq10$ decreases more than 2.5 times for $\xi/\xi_{L} = 0.3$ and
$\simgt$4.5 times for $\xi/\xi_{L} = 1$. We argue that "21 cm forest"
absorptions might be a powerful probe of the presence of decaying dark matter
in the early Universe.
