The present invention provides a positive-working photoresist composition comprising an alkali-soluble resin and a 1,2-naphthoquinonediazide-5-(and/or -4-)sulfonic ester of a specific water-insoluble alkali-soluble low molecular compound, wherein the high-performance liquid chromatography of the ester with an ultraviolet ray at 254 nm shows that the patterns corresponding to the diester components and complete ester component of the ester each fall within the specific range and a positive-working photoresist composition comprising a water-insoluble alkali-soluble resin, a water-insoluble alkali-soluble low molecular compound, and an ionization-sensitive radioactive compound which comprises a mixture of a naphthoquinone-diazidesulfonic diester of a water-insoluble alkali-soluble low molecular compound having three phenolic hydroxyl groups as an ionization-sensitive radioactive compound (A) and a naphthoquinonediazidesulfonic diester of a water-insoluble alkali-soluble low molecular compound having four phenolic hydroxyl groups as an ionization-sensitive radioactive compound (B) in an amount of 30% or more by weight based on the total amount of the ionization-sensitive radioactive compound.