PURPOSE: To carry out the hydrogenation without clogging of any catalyst layer or increase in pressure loss while easily controlling temp. by contacting hydrogen and a liquid fuel with the cotalyst layer formed by arranging a catalyst packed in a wire net in parallel to the passage of the hydrogen and liquid oil.

CONSTITUTION: A mixture of a liquid oil and hydrogen is supplied through a top pipe line of a reactor 2 to the reactor 2 including a catalyst layer 1 which is formed by providing a catalyst package filled with granular catalyst in vertical multiple layers and in parallel to the stream of the liquid oil and hydrogen in a platelike or columnlike structure whose entire surface is formed of a wire net, or within a wall of a honeycomb-like, or tubular structure whose wall is formed of double wire nets. The liquid oil is subjected to in the catalyst layer 1 by the action of the hydrogen and catalyst to prepare light oil and hydrocarbon gas. The hydrogen gas, and the obtd. light oil and hydrocarbon gas are discharged via a bottom pipe line 4 of the reactor 2 out of the system.