A chemically amplified positive photoresist composition for thick film that is used for forming a thick-film photoresist layer with a film thickness of 10 to 150 .mu.m on top of a support, comprising (A) a compound that generates acid on irradiation with active light or radiation, (B) a resin that displays increased alkali solubility under the action of acid, and (C) an alkali-soluble resin, wherein the component (B) comprises a resin formed from a copolymer containing a structural unit (b1) with a specific structure.