There is provided a positive photoresist composition for use in a method of forming a resist pattern that includes an immersion lithography step, which exhibits little line edge roughness and an excellent resist pattern profile. This photoresist composition includes a resin component (A), an acid generator component (B), an organic solvent (C), and a nitrogen-containing organic compound (D) represented by a general formula (1) shown below: [wherein, X, Y, and Z each represent, independently, an alkyl group that may contain an aryl group bonded to a terminal (although two terminals from the groups X, Y, and Z may also be bonded together to form a cyclic structure), at least one of X, Y, and Z contains a polar group, and the molecular weight of the compound (D) is at least 200].