Disclosed is a positive working photoresist composition comprising an acid-decomposable polymer containing at least a repeating unit represented by the following formula (I) and at least either one repeating unit represented by the following formula (IIa) or (IIb), and also disclosed is a positive working photoresist composition comprising (A) a compound capable of generating an acid upon irradiation with actinic rays or radiation, (B) an acid-decomposable polymer containing at least a repeating unit represented by the following formula (I) and at least either one repeating unit represented by the following formula (IIa) or (IIb), (C) at least one solvent capable of dissolving the components (A) and (B), (D) an organic basic compound and (E) at least one surfactant selected from fluorine-containing surfactants, silicon-containing surfactants and nonionic surfactants: