  Detectability of isolated black holes (IBHs) without a companion star but
emitting X-rays by accretion from dense interstellar medium (ISM) or molecular
cloud gas is investigated. We calculate orbits of IBHs in the Galaxy to derive
a realistic spatial distribution of IBHs, for various mean values of kick
velocity at their birth $\upsilon_{\rm avg}$. X-ray luminosities of these IBHs
are then calculated considering various phases of ISM and molecular clouds, for
a wide range of the accretion efficiency $\lambda$ (a ratio of the actual
accretion rate to the Bondi rate) that is rather uncertain. It is found that
detectable IBHs mostly reside near the Galactic Centre (GC), and hence taking
the Galactic structure into account is essential. In the hard X-ray band, where
identification of IBHs from other contaminating X-ray sources may be easier,
the expected number of IBHs detectable by the past survey by NuSTAR towards GC
is at most order unity. However, 30--100 IBHs may be detected by the future
survey by FORCE with an optimistic parameter set of $\upsilon_{\rm avg} = 50 \
\mathrm{km\ s^{-1}}$ and $\lambda = 0.1$, implying that it may be possible to
detect IBHs or constrain the model parameters.
