In accordance with the invention, the organo-aluminum compound to be used for this purpose is a compound expressed by a generic formula of AlR14 n X3 3-n (wherein R14 represents a hydrocarbon group of carbon number 1 to 20; X3 a halogen atom selected from the group consisting of fluorine, chlorine, bromine and iodine or a hydrogen atom; and n a real number of 0<n???3).