Proposed is a chemical-sensitization positive-working photoresist composition for photolithographic patterning in the manufacture of semiconductor devices having high transparency even to ultraviolet light of very short wavelength such as ArF excimer laser beams of 193 nm wavelength to exhibit high photosensitivity and capable of giving a patterned resist layer with high pattern resolution. The composition comprises (A) a resinous ingredient which is subject to an increase of the solubility in an aqueous alkaline developer solution in the presence of an acid and (B) a radiation-sensitive acid-generating compound. Characteristically, the resinous ingredient as the component (A) is a (meth)acrylic copolymer of which from 20% to 80% by moles of the monomeric units are derived from a (meth)acrylic acid ester of which the ester-forming group has a specific oxygen-containing heterocyclic ring structure.