A catalyst of the invention includes an imide compound having a N-substituted cyclic imide skeleton represented by following Formula (I): ##STR00001## wherein R is a hydroxyl-protecting group. Preferred R is a hydrolyzable protecting group. R may be a group obtained from an acid by eliminating an OH group therefrom. Such acids include, for example, carboxylic acids, sulfonic acids, carbonic acid, carbamic acid, sulfuric acid, nitric acid, phosphoric acids and boric acids. The catalyst may include the imide compound and a metallic compound in combination. In the presence of the catalyst, (A) a compound capable of forming a radical is allowed to react with (B) a radical scavenging compound and thereby yields an addition or substitution reaction product of the compound (A) and the compound (B) or a derivative thereof. This catalyst can produce an organic compound with a high selectivity in a high yield as a result of, for example, an addition or substitution reaction under mild conditions.