The present invention provides a positive resist composition capable of forming a resist pattern with high resolution, and a method of forming a resist pattern.This composition is a positive resist composition including a resin component (A) which exhibits increased solubility in an alkali developing solution under action of acid, and an acid-generator component (B) which generates acid upon irradiation, the resin component (A) containing a polymer including: a core portion represented by general formula (1) [Chemical Formula 1] PX—Y)a   (1) wherein P represents an a-valent organic group; a represents an integer of 2 to 20; Y represents an arylene group or an alkylene group of 1 to 12 carbon atoms; and X represents a specific linking group which can be cleaved under action of acid, and arm portions that are bonded to the core portion and are also composed of a polymer chain obtained by an anionic polymerization method.