A resist composition including a resin component having a constitutional unit derived from a compound represented by General Formula (a0-1) or a constitutional unit derived from a compound represented by General Formula (a0-2). In the formulae, R.sup.01 and R.sup.02 represent a hydrogen atom; L.sup.01 and L.sup.02 represent a single bond or a divalent linking group; Ar.sup.01 and Ar.sup.02 represent an aryl group substituted with iodine atoms; Xa.sup.01 represents a group which forms an alicyclic hydrocarbon group together with a carbon atom to which Ar.sup.01 is bonded; Ra.sup.02 represents a hydrocarbon group which may have a substituent; and Xa.sup.02 represents a group which forms an alicyclic hydrocarbon group together with a carbon atom to which Ra.sup.02 is bonded and a group which is bonded to Ar.sup.02 to form a condensed ring

##STR00001##

An onium salt consisting of a sulfonic acid anion having an aromatic sulfonic acid structure substituted with an aromatic ring-containing hydrocarbyl group and a triarylsulfonium cation containing iodine and fluorine generates an acid with limited diffusion. A chemically amplified positive resist composition comprising the onium salt as an acid generator and a polymer adapted to be decomposed under the action of acid to increase its solubility in alkaline developer is processed by lithography to form a pattern of rectangular profile.

An onium salt consisting of a sulfonic acid anion having an aromatic sulfonic acid structure substituted with an aromatic ring-containing hydrocarbyl group and a triarylsulfonium cation containing iodine and fluorine generates an acid with limited diffusion. A chemically amplified positive resist composition comprising the onium salt as an acid generator and a polymer adapted to be decomposed under the action of acid to increase its solubility in alkaline developer is processed by lithography to form a pattern of rectangular profile.