PURPOSE: To obtain the titled compound in high selectivity and in high yield, by using a catalyst prepared by combining a cobalt compound with a diphosphite compound and an organoaluminum compound, reacting ethylene with a 1,3-diene catalytically so that they are codimerized.

CONSTITUTION: Ethylene is reacted with a 1,3-diene in a solvent or in the absence of a solvent at room temperatureW 120°C under ethylene pressure of 3W50kg/cm2G in the presence of a catalyst consisting of a cobalt compound, a diphosphite compound shown by the formulaI(R1WR4 are alkyl, aryl, aralkyl, alkoxy, aryloxt, or halogen; R5 is alkylene, phenylene, biphenylene, a group shown by the formula II, etc.; with the proviso that R1 and R3 may form a bridge of -O-R6-O- and R2 and R4 may form a bridge of -OR7-O-, R6 and R7 are shown as for R5) and an organoaluminum compound shown by the formula III(R is alkyl, aryl, or aralkyl; X is R or H).