An onium salt compound having a cation moiety of the following formula (1) is disclosed. ##STR00001## wherein A represents I or S, m is 1 or 2, n is 0 or 1, x is 1 10, and Ar1 and Ar2 are (substituted) aromatic hydrocarbon group, and P represents --O--SO.sub.2R, --O--S(O)R, or --SO.sub.2R, wherein R represents a hydrogen atom, a (substituted) alkyl group, or a (substituted) alicyclic hydrocarbon group. The onium salt compound is suitable as a photoacid generator for photoresists of a positive-tone radiation-sensitive resin composition. The positive-tone radiation-sensitive resin composition containing this compound is useful as a chemically-amplified photoresist exhibiting high resolution at high sensitivity, responsive to various radiations, and having outstanding storage stability.