To provide an industrially advantageous photooxidation desulfurization catalyst for a fuel oil capable of efficiently oxidizing and removing a hardly removable organic sulfur compound contained in the fuel oil, particularly dibenzothiophenes.

The photooxidation catalyst is a photooxidation catalyst for a sulfur compound contained in the fuel oil containing a poly-acid compound represented by the general formula (1): (wherein [MxOy]n- represents a poly-acid anion, M represents a polyatom of which a part is sometimes substituted by a heteroatom, x and y represent an integer of 1 or more determined by the kind of M, [A]n+ represents an atom or an atomic group forming a pair ion of the poly-acid anion and n represents the number of valence).