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.