PURPOSE: To selectively remove a polycyclic aromatic compd. from a lubricant distillate by bringing the distillate into contact with a selective membrane under a perstraction condition.

CONSTITUTION: A distillate (initial b.p.; 350°C or higher) of a mixture of polycyclic aromatic hydrocarbon (dicyclic, tricyclic and tetracyclic aromatic compds. each containing below 75 mol.% of an aromatic molecule and having one or more alkyl group), a monocyclic aromatic compd. and a non-aromatic compd. is brought into contact with polyurea/urethane supported on a selective separation membrane, for example, a support composed of Teflon (R), etc., under a condition to permeate and separate the polycyclic aromatic compd.