To provide a catalyst system establishing high transition rate, selectively catalyzing isomerization reaction, usable at a low concentration by the formation of only a small amount of by-product and resistant to corrosion.

The objective catalyst is a composition produced by mixing an iron compound selected from an iron carbonyl of the formula: Fe(CO)5, Fe2(CO)9 or Fe3(CO)12, an anhydrous iron salt of the formula: FemXn ((m) is 1 or 2; (n) is 2 or 3; and X is a halide, carbonate, nitrate, nitrite, sulfide, sulfate, phosphate, thiocyanate, acetate or acetylacetate) or a mixture of two or more of the above compounds with a cyclopentadiene derivative of general formula I [R1 to R5 are each H, a 1-12C alkyl, a 5-8C cycloalkyl (the adjacent groups may together form a 3-14C ring) or SiR6R7R8 (R6 to R8 are each a 1-4C alkyl, a 5-8C cycloalkyl or a 6-14C aryl)] and 1,4-dichloro-2-butene and/or 3,4- dichloro-1-butene or their mixture at 40-180°C.