A process for producing organic sulfur acid or a salt thereof of the present invention allows an organic substrate to react with a sulfur oxide in the presence of a metallic compound catalyst and in the absence of N-hydroxy and N-oxo cyclic imide compounds and thereby yields a corresponding organic sulfur acid or a salt thereof. Such organic substrates include, for example, (a) homocyclic or heterocyclic compounds having a methylene group, (b) compounds having a methine carbon atom, (c) compounds having a methyl group or methylene group at the adjacent position to an unsaturated bond, (d) non-aromatic heterocyclic compounds having a carbon-hydrogen bond at the adjacent position to a hetero atom, and (e) straight-chain alkanes. The sulfur oxide includes, for example, sulfur dioxide. This production process can efficiently produce an organic sulfur acid or a salt thereof under mild conditions.