A method for synthesizing an organometallic compound, which is usefully utilized as a catalyst for manufacturing polyolefins by ring-opening metathesis polymerization of an olefin having strain in a molecule such as dicyclopentadiene or synthesizing epothilones by ring-dosing metathesis reaction, by an efficient and low cost synthesis method using a starting material which is easily available due to relatively simple chemical structure, and further without any possibility of coexistence of a vinylhetero compound or a vinyl compound exchanged which tends to accompany as an impurity in the system in conventional methods, characterized by reacting a starting material comprising a zero-valent transition metal complex (A) or a polyvalent transition metal complex (A'), a compound (B) or (B') shown by the following general formula (1) or (4), respectively, and a neutral ligand (C) or (C'), in one step under non-reducing condition or reducing condition. R.sup.1Y.sup.1CR.sup.2X.sup.1.sub.2 (1) R.sup.4Y.sup.2CR.sup.5X.sup.2.sub.2 (4)