PROBLEM TO BE SOLVED: To provide a fuel oil base material, in which a specular odor caused by olefin or dienes can be reduced and which can exhibit excellent oxidation stability and low-temperature fluidity when incorporated in a heavy oil composition, and to provide the heavy oil composition which contains the fuel oil base material and has excellent oxidation stability and low-temperature fluidity.SOLUTION: The fuel oil base material is obtained by mixing 1-50 parts volume of pyrolysis oil, which is obtained by pyrolyzing naphtha and has specific properties, with 100 parts volume of heavy naphtha and/or kerosene and hydrotreating an obtained mixture at 160-230°C reaction temperature. The heavy oil composition contains the fuel oil base material equal to or more than 1 vol.% and has 35 or higher cetane index, 60°C or higher flash point, minus 6°C or lower plugging point and minus 20°C or lower flow point.
