Abstract (704) Interamnia is one of the largest asteroids located 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 wave bands and utilize a thermophysical model for realistic surface layers to analyze mid-infrared data from Subaru along with those of IRAS, AKARI, and Wide-field Infrared Survey Explorer (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 μ m, with a roughness of 0.30 − 0.17 + 0.35 and rms slope of 27 − 9 + 13 deg, thereby producing thermal inertia ranging from 9 to 92 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 km , respectively, being indicative of a bulk density of 1.86 ± 0.63 g cm −3 . The low thermal inertia is consistent with typical B/C-type asteroids with D ≥ 100 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 the southern and northern hemispheres appear to be very different. Finally, we present an estimation of volume fraction of water ice of 9%–66% from the published grain density and porosity of carbonaceous chondrites.