PURPOSE: To dimerize and codimerize advantageously the titled compound, by pretreating a nickel compound with an organoaluminum halide compound and the titled compound at low temperature of ≤a specific temperature to give a catalyst precursor, and heating it to ≥a specified temperature.

CONSTITUTION: A nickel compound is blended with an organoaluminum halide compound and a lower monoolefin at a low temperature of ≤10°C, usually -20W10°C, preferably -5W5°C while stirring (stirring speed usually at ≥500W1,000RPH) to prepare a catalyst precursor, and the lower monoolefin is dimerized or codimerized at ≥40°C. The formation of an insoluble nickel salt and/or metallic nickel which not only shows no catalytic activity but also attaches itself to a reactor and is accumulated to cause problems of clogging of plant or shortage of heat transfer is suppressed, and the catalytic activity to the reaction and economic efficiency are improved.