A top temperature T.sub.1 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 T.sub.2 is raised up to
300.degree. 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.