A method for forming a pattern of conjugated system polymer which can be given an electrical conductivity by doping treatment which comprises forming a thin film of a polymeric sulfonium salt having a recurring unit represented by the following general formula: ##STR1## wherein R.sub.1 represents a group capable of conjugating with an adjacent vinylene group formed by elimination of sulfonium salt as a side chain, R.sub.2 and R.sub.3 each represents hydrocarbon group having 1 to 20 carbon atoms, and X.sup.- represents counter ion, on a substrate, irradiating said thin film with a prescribed pattern of light including a deep ultraviolet ray having a wavelength not longer than 300 nm and then subjecting the thin film to development.