A polymer compound that, within a chemically amplified positive resist system, exhibits a significant change in alkali solubility from a state prior to exposure to that following exposure, as well as a photoresist composition that includes such a polymer compound and a method for forming a resist pattern, which are capable of forming fine patterns with a high level of resolution. The polymer compound includes, as an alkali-soluble group (i), a substituent group in which a group selected from amongst alcoholic hydroxyl groups, carboxyl groups, and phenolic hydroxyl groups is protected with an acid dissociable, dissolution inhibiting group (ii) represented by a general formula (1) shown below: [Formula 1] —CH2—O-AO—CH2—]n  (1) (wherein, A represents an organic group of 1 to 20 carbon atoms with a valency of at least n+1, and n represents an integer from 1 to 4).