ycarboxylic acid ester in an amount of 0.1 to 1 mol and the titanium compound in an amount of 0.1 to 200 mol, both based on 1 mol of the magnesium compound, (B) an organoaluminum compound catalyst component represented by the following formula: R1 m A1(OR2)n Hp Xq wherein R1 and R2, which may be the same or different, are each a hydrocarbon group having 1 to 15 carbon atoms, X is a halogen atom, a