A sulfonic acid anion-containing polymer having an alkylsulfonium cation not in covalent bond thereto can be readily prepared by reacting a sulfonic acid anion-containing polymer having an ammonium or metal cation with an alkylsulfonium salt under mild conditions. A resist composition comprising the inventive polymer is effective for suppressing acid diffusion since the sulfonium salt is bound to the polymer backbone. When processed by the ArF lithography, the polymer exhibits a lower absorption at the exposure wavelength than the triarylsulfonium salt form PAGs, resulting in improved resolution, mask fidelity, and LWR.