PURPOSE: To produce polypropylene in high yield, by polymerizing propylene containing dissolved organo-Al compound, in the presence of a catalyst composed of the organo-Al compound and a Ti halide supported on a carrier containing a Mg halide.

CONSTITUTION: A magnesium halide such as magnesium chloride and a titanium halide such as titanium trichloride are co-crushed to obtain a carrier, and is used in combination with an organo-aluminum compound such as triethylaluminum as a catalyst. An organo-aluminum compound such as triethylaluminum is dissolved in propylene having a purity of 95% at a concentration of preferably 1W2,500ppm, and the propylene is polymerized by vapor-phase polymerization or bulk polymerization to obtain the objective polypropylene.