PURPOSE: To obtain the titled high-purity compound useful as a polymerization regulator, in high selectivity safely, by dimerizing α-methylstylene in the presence of an organic compound selected from an aliphatic monoether, etc. in the reaction system using acid clay as a catalyst.

CONSTITUTION: α-Methylstylene is reacted and dimerized using acid clay or activated clay as a catalyst in the presence of one or more organis compounds selected from an aliphatic monoether, aliphatic mixed ether, and cyclic ether in the reaction system at 50W120°C, to give the desired compound. An aliphatic monoether, e.g., propyl ether, butyl ether, aliphatic mixed ether, e.g., methyl butyl ether, ethyl butyl ether, etc., and cyclic ether, e.g., 1,4-dioxane, etc. is used as the organic compound. The amount of the catalyst used is 0.5W20wt% based on α-methyl- stylene and the amount of the ether used is 1W100wt% based on the catalyst.