The invention has an object of safely and simply preparing a large amount of a ruthenium metathesis catalyst, which is used as a catalyst for a carbon-carbon bond formation using, particularly, a metathesis reaction. The metathesis catalyst has the following complex composition (A) or (B). The composition (A) includes RuX12(arene)(PR1R2R3) and R4CHX22, R5C=CH or R4CHX2 and a reducing agent, wherein X1 and X2 respectively are a halogen atom; arene is a hydrocarbon having a benzene ring; R1, R2 and R3, which may be the same or different, respectively are an alkyl group having 1-8 carbon atoms, a cycloalkyl group having 3-8 carbon atoms or an optionally substituted aryl group, wherein the substituent group is an alkyl group having 1-8 carbon atoms, an alkoxy group having 1-8 carbon atoms, an alkylamino group having 1-8 carbon atoms or a halogen atom; R4 is an alkyl group which has 1-8 carbon atoms and may have an ether bond or an ester bond, an optionally subsituted aryl group, wherein the substituent group is a halogen atom or a hydroxyl group; or cycloalkyl group having 3-8 carbon atoms; and R5 is an optionally substituted alkyl group which has 1-8 carbon atoms and may have an ether bond or an ester bond, wherein the substituent group is a halogen atom or a hydroxyl group, an aryl group or a cycloalkyl group having 3-8 carbon atoms. The composition B includes [RuX12(arene)]2, PR1R2R3, R5C=CH or R4CHX2 and a reducing agent, wherein X1, arene, R1, R2, R3, R4 and R5 are the same as defined above.