Coal is the most abundant fuel on earth, and low-rank coal (LRC) such as sub-bituminous coal and lignite makes up about half of all coal deposits. LRC is inconvenient to use due to its low caloric value and high content of moisture together with the strong tendency of spontaneous combustion due to these oxygen rich coals, etc. Solving these problems would substantially improve the efficiency of LRC usage. In this study, we describe a drying technique utilizing hot oil immersion. This upgrading process may be executed easily under relatively low temperature condition due to the difference of heat capacity and thereby pressure gradient between coal and heating oil. This results in greatly reducing its energy cost. Drying tests of Indonesian lignite were performed with refined waste oil and B-C heavy oil, which were heated to $120^{circ}C$, $130^{circ}C$ or $140^{circ}C$. Following 10 min of treatment, the moisture content of the upgraded coal was improved from 32% to 2.0-3.2%, and its high heating value from 3,000 kcal/kg to 6,000 kcal/kg.