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The ultraluminous infrared galaxy IRAS 17208$-$0014 is a late-stage mergerthat hosts a buried active galactic nucleus (AGN). To investigate its nuclearstructure, we performed high spatial resolution($\sim0.\!\!^{\prime\prime}04\sim32\,\mathrm{pc}$) Atacama LargeMillimeter/submillimeter Array (ALMA) observations in Band 9($\sim$450\,\micron\ or $\sim$660\,GHz), along with near-infrared AKARIspectroscopy in 2.5--5.0\,\micron. The Band 9 dust continuum peaks at the AGNlocation, and toward this position CO($J$=6--5) and CS($J$=14--13) are detectedin absorption. Comparison with non-local thermal equilibrium calculationsindicates that, within the central beam ($r\sim20\,\mathrm{pc}$), there existsa concentrated component that is dense ($10^7\,\mathrm{cm}^{-2}$) and warm($>$200\,K) and has a large column density($N_\mathrm{H_2}>10^{23}\,\mathrm{cm}^{-2}$). The AKARI spectrum shows deep andbroad CO ro-vibrational absorption at 4.67\,\micron. Its band profile is wellreproduced with a similarly dense and large column but hotter ($\sim$1000\,K)gas. The region observed through absorption in the near-infrared is highlylikely in the nuclear direction, as in the sub-millimeter, but with a narrowerbeam including a region closer to the nucleus. The central component isconsidered to possess a hot structure where vibrationally excited HCN emissionoriginates. The most plausible heating source for the gas is X-rays from theAGN. The AKARI spectrum does not show other AGN signs in 2.5--4\,\micron, butthis absence may be usual for AGNs buried in a hot mid-infrared core. Besides,based on our ALMA observations, we relate various nuclear structures of IRAS17208$-$0014 that have been proposed in the literature.

Extremely Buried Nucleus of IRAS 17208$-$0014 Observed at Sub-Millimeter and Near-Infrared Wavelengths