Process for hydrocracking heavy hydrocarbon oils and catalyst therefor

A process for hydrocracking a heavy hydrocarbon oil, comprising contacting the heavy hydrocarbon oil with a catalyst at a temperature of 420.degree.-480.degree. C., a hydrogen pressure of 100-250 Kg/cm.sup.2 and a liquid hourly space velocity of 0.1-10 hr.sup.-1, said catalyst including a porous inorganic carrier composed mainly of alumina or titania, and two or more catalytic metal components composed with said carrier, the metals of said catalytic metal components being either (a) V and at least one first auxiliary element selected from the group consisting of Zn, Fe, Co, Ni, Cu, Ag, Sn and Pb or (b) Mo and at least one second auxiliary metal component selected from the group consisting of Zn, Cu, Ag, Sn, Pb and the lanthanum-series elements, and said catalyst having at least 60% of its total pore volume in pores with a diameter of 200 .ANG. or more, at least 40% of its total pore volume in pores with a diameter of 300 .ANG. or more and not more than 20% of its total volume in pores with a diameter of at least 1000 .ANG..