A so-called Ziegler catalyst composed of a titanium halide-containing solid ingredient and an organoaluminum compound for the polymerization of olefins can be imparted with greatly improved storage stability by first subjecting the catalytic composition to a pre polymerization treatment and then to a contacting treatment with carbon dioxide and/or carbon monoxide. The catalytic activity is almost unchanged even after storage of the stabilized catalyst for 90 days or longer in nitrogen gas. The stabilized catalyst can also be used advantageously in a continuous-process polymerization of propylene by continuously introducing the monomer carrying the catalyst owing to absence of blocking of the pipe line with the polymer formed in-line.