PURPOSE: To prolong the operating time of the equipment by hydrotreating a heavy oil in a fixed-bed reaction tower and hydrotreating the effluent in a suspended-bed reaction tower.

CONSTITUTION: A heavy oil feedstock is introduced into fixed-bed reaction towers 1, 2 and 3 each of which is packed with a hydrotreatment catalyst and hydrotreated under conditions of a reaction temperature of 320-410°C, a hydrogen pressure of 50-250kg/cm2, a liquid hourly space velocity of 0.1-2.0hr-1 and a hydrogen/oil ratio of 300-1200nM3/kl to obtain a hydrotreated oil having a degree of demetallization of at most 80wt.% for V and Ni. This oil is introduced into a suspended-bed reaction tower 4 containing a hydrotreatment catalyst and hydrotreated under conditions to give a degree of demetallization, a degree of desulfurization and a degree of denitrogenation all of which are higher than those attained in the former step and are based on the feedstock oil.