To reduce the deactivation of a catalyst caused by coke thereby prolonging the life of the catalyst and extending the operation time for hydrotreatment by bringing a feedstock hydrocarbon oil into contact with an adsorbing agent before the hydrotreatment to adsorptively remove and decrease polycyclic aromatic compounds in the hydrocarbon oil and then carrying out hydrotreatment in the presence of a catalyst.

As a feedstock hydrocarbon oil, crude oils, vacuum residues, topped crudes, catalyst cracking gas oils, thermal cracking gas oils and the like can be cited. Polycyclic aromatic compounds in the feedstock hydrocarbon oil are adsorptively removed to decrease to not more than 60 wt.%, preferably not more than 40 wt.% of the initial content. As an adsorbing agent, active carbon, alumina, silica-alumina, zeolite and the like can be exemplified. A preferable adsorbing agent contains at least 0.15 mmol/g of a solid acid (measured by the ammonia adsorption method), and silica-alumina containing 20-40 wt.% of alumina is preferable. Hydrotreatment can be carried out using an ordinary hydrotreatment apparatus and an ordinary hydrotreatment catalyst.