PURPOSE: To perform hydrotreating of heavy oil economically without using any catalyst regeneration column by suspending in the heavy oil a fresh direct desulfurization catalyst and/or a waste direct desulfurization catalyst, subjecting the suspension to hydrotreating, and recovering the catalyst(s) by means of a decanter for reuse in hydrotreating.

CONSTITUTION: A metal selected from among Mo, Co, Ni and W is supported on an aluminum carrier to obtain a fresh direct desulfurization catalyst with an average grain size of 3 to 40μm. A waste catalyst removed from a direct desulfurization apparatus that directly desulfurizes a residue such as a residue of atmospheric or vacuum distillation is subjected to a regeneration treatment where the carbon sticking to its surface is burned off to thereby obtain a waste direct desulfurization catalyst with an average grain size of 3 to 40μm and a specific surface area of 40 to 200m2/g, in which 0.7 to 20wt.% vanadium and 0.2 to 10wt.% nickel are accumulated. In heavy oil, the fresh and/or the waste catalyst are suspended in an amount of 0.05 to 10.0wt.% based on the amount of the heavy oil. the suspension is then subjected to hydrotreating for 10 minutes to 4 hours at a temperature of 400 to 480°C and a pressure of 50 to 300kg/cm2.G, at a liquid hourly space velocity of 0.1 to 3.0hr-1, at a hydrogen partial pressure of 35 to 200kg/cm2, and with a hydrogen consumption of 50 to 300m2 per kiloliter of oil. The used catalyst(s) is (are) removed from the mixture by means of a decanter, and recycled for hydrotreating.