A catalyst of the invention includes a cyclic acylurea compound having a cyclic acylurea skeleton represented by following Formula (I): ##STR00001## wherein R is a hydrogen atom or a hydroxyl-protecting group; n is 1 or 2; G is a carbon atom or a nitrogen atom, where two Gs are the same or different when n is 2. The catalyst may include the cyclic acylurea 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.