The present invention provides a chemically amplified positive resist composition comprising (i) a polymer which is insoluble or poorly soluble in an alkali aqueous solution but becomes soluble in an alkali aqueous solution by the action of an acid, (ii) an acid generator, and (iii) a compound of the formula (I). ##STR00001## The present invention also provides an ester derivative useful as a component of the chemically amplified positive resist composition, and process for producing the ester derivative.