  We investigate a possibility to find an accreting isolated black hole (IBH)
with mass $1-100~\mathrm{M}_{\odot}$ within Central Galactic Molecular Zone
(CMZ) in the submillimetre and IR spectral range with help of planned space
observatories James Webb Space Telescope (JWST) and Millimetron (MM). We assume
the spherical mode of accretion. We develop the simplest possible quantitative
model of the formation of radiation spectrum in this range due to synchrotron
emission and show that it fully agrees with the more complicated model of Ipser
and Price 1982 for expected values of accretion rate.
  If a substantial fraction of LIGO events was caused by merger of primordial
black holes, the JWST would be able to find them provided that there is a cusp
in distribution of dark matter in the Galaxy and that the accretion efficiency
parameter $\lambda $ defined as the ratio of accretion rate onto IBH to its
Bondi-Hoyle-Lyttleton value is larger than $\sim 10^{-2}$. A comparable amount
of IBHs is also predicted by recent models of their formation due to stellar
evolution. MM capabilities are hampered by the effect of confusion due to
distant submillimetre galaxies, but it can also be used for such purposes if
the confusion effect is properly dealt with. In case of efficient accretion
with $\lambda \sim 1$, both instruments could detect IBHs even when their
number densities are as low as $10^{-6}~\mathrm{pc}^{-3}$.
