PURPOSE: To provide a novel method for treating raw material oil containing furfural and a petroleum purified middle distillate.

CONSTITUTION: In a method for reducing the content of a furfural of a raw material containing furfural and a hydrocarbon middle distillate, the raw material is brought into contact with a separation membrane composed of a non- porous polyimine separation layer crosslinked by a crosslinking agent of polyisocyanate or poly(carbonyl chloride) and a pressure drop is kept between both surfaces of the membrane to form high pressure side residue having an increased of the hydrocarbon middle distillate and a reduced content of furfural and a low pressure permeated matter with a reduced content of the hydrocarbon middle distillate and with an increased content of furfural and, by keeping the pressure on the low pressure discharge side of the membrane higher than the vapor pressure of the permeated matter, the permeated matter is retained in a liquid phase and, by keeping the pressure on the high pressure residue side of the membrane higher than the vapor pressure of the residue, the residue is retained in a liquid phase.