  A large fraction of baryons predicted from the standard cosmology has been
missing observationally. Although previous numerical simulations have indicated
that most of the missing baryons reside in large-scale filaments in the form of
Warm Hot Intergalactic Medium (WHIM), it is generally very difficult to detect
signatures from such a diffuse gas. In this work, we focus on the hyperfine
transition of neutral hydrogen (HI) called 21-cm line as a tool to trace the
WHIM. For the purpose, we first construct the map of the 21-cm signals by using
the data provided by the state-of-the-art cosmological hydrodynamics simulation
project, Illustris, in which detailed processes affecting the dynamical and
thermal evolution of the WHIM are implemented. From the comparison with the
constructed 21-cm signal map with the expected noise level of the Square
Kilometre Array phase 1 mid-frequency instrument (SKA1-mid), we find that the
21-cm signals from the filamentary structures at redshifts z=0.5-3 are
detectable with the SKA1-mid if we assume the angular resolution of
\Delta\theta > 10 arcmin and the observing time of t_obs > 100 hours. However,
it also turns out that the signals mainly come from galaxies residing in the
filamentary structures and the contribution from the WHIM is too small to
detect with the SKA1-mid. Our results suggest that about 10 times higher
sensitivity than the SKA1-mid is possibly enough to detect the WHIM at z=0.5-3.
