The present invention provides a sulfonium salt of the formula (Ia) ##STR00001## a polymeric compound comprising a structural unit of the formula (Ib) ##STR00002## and a chemical amplification type positive resist composition comprising (A) an acid generator comprising at least one compound selected from the group consisting of a sulfonium salt of the formula (Ia), a polymeric compound comprising a structural unit of the formula (Ib), and a sulfonium salt of the formula (Ic); and (B) resin which contains a structural unit having an acid labile group and which itself is insoluble or poorly soluble in an alkali aqueous solution but becomes soluble in an alkali aqueous solution by the action of an acid.