Process for producing polypropylene or propylene random copolymers

The invention provides a process for producing polypropylene and/or random copolymers of propylene type using a stereoregular catalyst which comprises a titanium-containing solid catalyst component (A), an organoaluminum compound (B) and if required an organosilicon compound (C), wherein [I] before carrying out homopolymerization of propylene or random copolymerization of propylene with an alpha-olefin other than propylene, the above component (A), an organoaluminum compound (B') and if required an organosilicon compound (C') are used, and a small amount of propylene or ethylene and/or an alpha-olefin of 4 to 20 carbon atoms for pre-activation treatment is supplied to the above component (A), and [II] a catalyst which is prepared by adding an organosilicon compound (C"") to the pre-activated stereoregular catalyst in a molar ratio of 0.1 to 50 as Si/Ti, is used to produce polypropylene (propylene homopolymer) or propylene random copolymers (propylene-alpha-olefin random copolymers) with lesser formation of lump and string-like irregular shape polymers.