A positive resist composition and a pattern forming method using the resist composition are provided, the resist composition including: (A) a resin containing a repeating structural unit represented by formula (I) as defined in the specification and being capable of decomposing by an action of an acid to increase the solubility in an alkali developer; (B) an acid generator; and (C) a mixed solvent containing at least one solvent selected from the group consisting of the following Group (a) and at least one solvent selected from the group consisting of the following Groups (b) to (d): Group (a): an alkylene glycol monoalkyl ether, Group (b): an alkylene glycol monoalkyl ether carboxylate, Group (c): a linear ketone, a branched chain ketone, a cyclic ketone, a lactone and an alkylene carbonate, and Group (d): a lactic acid ester, an acetic acid ester and an alkoxypropionic acid ester.