To provide a method for producing an organosulfur oxide in a manner excellent in workability and operability, low in cost, and excellent in mass productivity and production output, in which an organosulfur compound such as a dibenzothiophene is selectively oxidized into a polar organosulfur oxide such as a sulfoxide or sulfone derivative within a short time, and the organosulfur compound is effectively separated from a hydrocarbon oil to produce the desired useful organosulfur oxide in a high purity and a high recovery rate.

The method comprises the step of treating a mixture containing a hydrocarbon oil, a solvent, and an oxidizing agent under supercritical or subcritical high-temperature and high-pressure conditions of the organic solvent to dissolve an organosulfur oxide obtained by oxidizing an organosulfur compound contained in the hydrocarbon oil in the solvent, the step of separating the sulfur-containing solvent in which the organosulfur oxide is dissolved from the hydrocarbon oil, and the sulfur oxide recovery step of purifying and recovering the organosulfur oxide dissolved in the sulfur-containing solvent.

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