A positive tone radiation-sensitive resin composition comprising (A) a 1-substituted imidazole, (B) a photoacid generator, and (C-a) a resin protected by an acid-dissociable group, insoluble or scarcely soluble in alkali, but becoming soluble in alkali when the acid-dissociable group dissociates or (C-b) an alkali-soluble resin and an alkali solubility controller, and a negative tone radiation-sensitive resin composition comprising (A), (B), (D) an alkali-soluble resin, and (E) a compound that can crosslink the alkali-soluble resin in the presence of an acid. The radiation-sensitive resin composition of the present invention is a chemically amplified resist exhibiting high resolution and high storage stability as a composition, and suitable for microfabrication sensible to active radiations, for example, ultraviolet rays such as g-lines and i-lines, deep ultraviolet rays represented by a KrF excimer laser, ArF excimer laser, F2 excimer laser, and EUV excimer laser, and electron beams.