PURPOSE: To obtain the titled substance useful as a precursor of the third component of an ethylene-propylene-diene polymer rubber, economically with little by-products, by terminating the reaction of cyclopentadiene with butadiene when the concentration of cyclopentadiene reaches within a specific range.

CONSTITUTION: Cyclopentadien and butadiene are charged to a reactor at a butadiene/cyclopentadiene molar ratio of 0.30W2.00, preferably 0.50W1.80, and are made to react with each other in a solvent such as benzene in the presence of a polymerization inhibitor such as hydroquinone, t-butycatechol, etc., at 100W150°C, preferably 120W140°C under 0W100kg/cm2G, especially 5W50kg/cm2G pressure. The concentration of cyclopentadien in the reaction system is inspected during the reaction, and when the concentration reaches 1.5W7.0wt%, preferably 2.0W4.0wt%, the reaction is terminated to obtain the objective compound.