A method of trimming a resist pattern, including forming a positive resist film on a substrate, the positive resist film is exposed and the positive resist film is subjected to an alkali development to form a first resist pattern having an alkali-insoluble region on the surface thereof; applying a resist trimming composition including an acid to the substrate on which the first resist pattern is formed; a heating the first resist pattern coated with the resist trimming composition, and the solubility of the first resist pattern in a developing solution is changed under action of the acid included in the resist trimming composition; and developing the first resist pattern after heating with an organic solvent to remove the alkali-insoluble region of the first resist pattern, the resist trimming composition including the acid and a solvent which does not dissolve the first resist pattern.