  (704) Interamnia is one of the largest asteroids that locates in the outer
main-belt region, which may contain a large amount of water ice underneath its
surface. We observe this asteroid using 8.2 m Subaru telescope at mid-infrared
wavebands, and utilize thermophysical model for realistic surface layers
(RSTPM) to analyze mid-infrared data from Subaru along with those of IRAS,
AKARI and WISE/NEOWISE. We optimize the method to convert the WISE magnitude to
thermal infrared flux with temperature dependent color corrections, which can
provide significant references for main-belt asteroids at a large heliocentric
distance with low surface temperature. We derive best-fitting thermal
parameters of Interamnia - a mean regolith grain size of $190_{-180}^{+460}~\rm
\mu m$, with a roughness of $0.30_{-0.17}^{+0.35}$ and RMS slope of
$27_{-9}^{+13}$ degrees, thereby producing thermal inertia ranging from 9 to
$92~\rm Jm^{-2}s^{-1/2}K^{-1}$ due to seasonal temperature variation. The
geometric albedo and effective diameter are evaluated to be
$0.0472_{-0.0031}^{+0.0033}$ and $339_{-11}^{+12}~\rm km$, respectively, being
indicative of a bulk density of $1.86\pm0.63~\rm g/cm^3$. The low thermal
inertia is consistent with typical B/C-type asteroids with $D\geq100$ km. The
tiny regolith grain size suggests the presence of a fine regolith on the
surface of Interamnia. Moreover, the seasonal and diurnal temperature
distribution indicates that thermal features between southern and northern
hemisphere appear to be very different. Finally, we present an estimation of
volume fraction of water ice of $9\%\sim66\%$ from the published grain density
and porosity of carbonaceous chondrites.
