PURPOSE: To expose the radiation sensitive composition to active chemical rays to form a latent image, to allow it to produce an acid at the parts of the latent image, and to render these parts developable with an aqueous solution of alkali by an acid-catalyzed reaction by using a specified medium.

CONSTITUTION: This composition contains a medium (A) having reactivity of enhancing solubility in an aqueous solution of alkali by the acid-catalyzed reaction and an alkyl onium salt B. As the medium (A), a polymer obtained by protecting the phenolic hydroxyl groups of an alkali-soluble phenol resin, such as polyvinylphenol, with tetrahydropyranyloxy-styrene, for example, a copolymer of p-hydropyranyloxy-styrene p-hydroxystyrene can be used, and as the medium (B), trifluoromethanesulfonic acid trimethylsulfonium and the like can be used.