PURPOSE: To obtain a polymerization catalyst which is useful for the (co) polymerization of an olefin and efficiently provides a uniform olefin polymer having a narrow molecular weight distribution by incorporating therein a specific compound and an activation promoter.

CONSTITUTION: The catalyst comprises a compound of the formula [(Ra)5X5] MYbZc [M is a metal of the Group 3-10 in the Periodic Table or a lanthanide; X is an element of the Group 14-15 in the Periodic Table, one or more of 5 Xs contain an elemet other than C, and 5 Xs have a total of 6 π atoms, constituting a monoanionic aromatic ligand; R is H, a halogen, a 1-20C hydrocarbon group, an O-containing group or an Si-containing group; a is 0 or 1; Y is a σ-bonding ligand or a chelating ligand; b is 0 or more and an integer equal to (the valency of M-1); Z is a Lewis base; and c is 0-3] [e.g. (2,3,4,5- tetramethylphosphoryl) titanium dimethoxide chloride] and an activation promoter (e.g. methylaluminoxane).