A positive resist composition comprising (a) a resin that is decomposed by the action of an acid to increase solubility in an alkali developing solution, contains a structural unit having a group represented by formula (X) defined in the specification, has a weight average molecular weight of not more than 5,000, and contains an acid decomposable group in an amount of not more than 40% based on the sum total of a number of the acid decomposable group and a number of an alkali-soluble group not protected with the acid decomposable group, and (b) a compound that generates an acid upon irradiation of an actinic ray or radiation.