A hydrodesulfurization catalyst for diesel oil which supports one or more metals selected from the group consisting of elements in Group 6 of the long periodic table, one or more metals selected from the group consisting of elements in Group 9 or 10 of the long periodic table, phosphorus, and an organic acid on a composite oxide support containing 80% by mass to 99.5% by mass of alumina, and 0.5% by mass to 20% by mass of HY zeolite, the catalyst comprising: 10% by mass to 40% by mass of one or more metals selected from the group consisting of elements in Group 6 in terms of an oxide based on the catalyst; 1% by mass to 15% by mass of one or more metals selected from the group consisting of elements in Group 9 or 10 in terms of an oxide based on the catalyst; 1.5% by mass to 8% by mass of phosphorus in terms of an oxide based on the catalyst; 0.8% by mass to 7% by mass of carbon derived from the organic acid in terms of an element based on the catalyst; and 0.2 moles to 1.2 moles of the organic acid per 1 mole of one or more metals selected from the group consisting of elements in Group 9 or 10 of the long periodic table, wherein the organic acid contains substantially no sulfur, and wherein a specific surface area of the hydrodesulfurization catalyst measured by a nitrogen adsorption method is from 110 m 2 /g to 300 m 2 /g, a pore volume of the hydrodesulfurization catalyst measured by a mercury penetration method is from 0.3 ml/g to 0.6 ml/g, an average pore diameter of the hydrodesulfurization catalyst measured by a mercury penetration method is from 6.5 nm to 14 nm, and the HY zeolite has (a) a SiO 2 /Al 2 O 3 (molar ratio) of 3 to 10, (b) a crystal lattice constant of 2.435 nm to 2.465 nm, as measured by X-ray diffraction apparatus as indicated in the description, (c) a molar ratio of Al in the zeolite framework to total Al of 0.2 to 0.9, and (d) a crystallite diameter of 30 nm to 100 nm, wherein the molar number of aluminum atoms in the zeolite framework to the total number of aluminum atoms of the zeolite is calculated from the molar ratio of SiO 2 /Al 2 O 3 by chemical composition analysis and the crystal lattice constant, as indicated in the description. 10% by mass to 40% by mass of one or more metals selected from the group consisting of elements in Group 6 in terms of an oxide based on the catalyst; 1% by mass to 15% by mass of one or more metals selected from the group consisting of elements in Group 9 or 10 in terms of an oxide based on the catalyst; 1.5% by mass to 8% by mass of phosphorus in terms of an oxide based on the catalyst; 0.8% by mass to 7% by mass of carbon derived from the organic acid in terms of an element based on the catalyst; and 0.2 moles to 1.2 moles of the organic acid per 1 mole of one or more metals selected from the group consisting of elements in Group 9 or 10 of the long periodic table, wherein the organic acid contains substantially no sulfur, and wherein a specific surface area of the hydrodesulfurization catalyst measured by a nitrogen adsorption method is from 110 m 2 /g to 300 m 2 /g, a pore volume of the hydrodesulfurization catalyst measured by a mercury penetration method is from 0.3 ml/g to 0.6 ml/g, an average pore diameter of the hydrodesulfurization catalyst measured by a mercury penetration method is from 6.5 nm to 14 nm, and the HY zeolite has (a) a SiO 2 /Al 2 O 3 (molar ratio) of 3 to 10, (b) a crystal lattice constant of 2.435 nm to 2.465 nm, as measured by X-ray diffraction apparatus as indicated in the description, (c) a molar ratio of Al in the zeolite framework to total Al of 0.2 to 0.9, and (d) a crystallite diameter of 30 nm to 100 nm, wherein the molar number of aluminum atoms in the zeolite framework to the total number of aluminum atoms of the zeolite is calculated from the molar ratio of SiO 2 /Al 2 O 3 by chemical composition analysis and the crystal lattice constant, as indicated in the description.