To provide a method of efficiently reforming oil in a low cost by selectively oxidizing heterocyclic compounds bearing condensed rings that exists in the oil through photoctalytic reactions, particularly dibenzothiophene that is very difficult to be desulfurized and removing the formed oxides, and to provide an efficient and low cost hydrosulfurization process using the reforming process.

As a pretreatment process for the hydrodesulfurization process for oil, the oil including heterocyclic compounds bearing condensed rings is irradiated with light in the presence of a low polar hydrocarbon solvent and a photocatalyst and the oxidized products of the heterocyclic compounds are dissolved in the solvent and removed whereby the oil is reformed.