A large-scale and efficient industrialization of olefin polymerization by metallocene catalysts is realized by, in olefin polymerization by metallocene-based catalysts, increasing catalytic activity and polymerization efficiency to sufficiently bring out the performances of the metallocene catalysts and maintain a stable polymerization reaction. The amount of a halogen-containing compound is controlled so that the gram-atomic amount ratio to a transition metal of a halogen atom in a halogen-containing compound in solvents and materials other than a halogen atom in a catalyst component is 0.8 or less, in manufacturing plants of commercial level in an olefin polymerization by a metallocene catalyst.