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A top temperature T1 of a distillation tower 1 is held below a liquefying temperature of a light fraction by returning a part of an exhaust gas W, which is cooled by a condenser 5, to the upper zone of the distillation tower 1. A bottom temperature T2 is raised up to 300 C. at highest by returning a part of a liquid product P from a re-boiler 3 to a lower zone of the distillation tower 1. When a liquid hydrocarbon L comes in countercurrent contact with a stripping gas G inside the distillation tower 1 with the temperature profile that an inner temperature gradually falls down along an upward direction, mercury is efficiently transferred from the liquid L to a vapor phase without effusion of the light fraction in accompaniment with the exhaust gas W.