Positive photoresist composition and method of forming resist pattern

A positive resist composition comprising (A) a resin ingredient which has acid-dissociable dissolution-inhibitive groups and comes to have enhanced alkali solubility by the action of an acid and (B) an acid generator ingredient which generates an acid upon exposure to light, wherein the resin ingredient (A) is a copolymer (A1) which comprises first structural units (a1) derived from hydroxystyrene and second structural units (a2) derived from a (meth)acrylic ester having an alcoholic hydroxy group and in which part of the hydroxy groups of the structural units (a1) and of the alcoholic hydroxy groups of the structural units (a2) have been protected by the acid-dissociable dissolution-inhibitive groups. The acid generator ingredient (B) comprises a diazomethane type acid generator and an onium salt type acid generator, or the composition further contains a compound which has at least one acid-dissociable dissolution-inhibitive group and can generate an organic carboxylic acid by the action of the acid generated from the ingredient (B).