PURPOSE: To prevent post polymerization and fish eye formation without causing any disadvantage to an apparatus, by adding a specified substance in carrying out a process comprising polymerizing propylene in liquid phase in the substantial absence of a diluent and evaporating and separating unreacted components in a low-pressure separation zone.

CONSTITUTION: Polypropylene is recovered (15) in the form of powder by polymerizing propylene in liquid phase in the substantial absence of a diluent with the aid of a catalyst system based on a Ti-containing solid catalytic component and an organoaluminum compound, introducing the resulting polypropylene slurry through a conduit 2 into a low-pressure separation zone 1 maintained at a pressure lower than the polymerization reaction pressure and evaporating and separating (via 7) unreacted propylene. Then, a carboxylate ester effective in inhibiting post polymerization is added. The amount of the carboxylate ester added is 1W50mol per g-atom Ti, and conduits for its introduction are indicated by 13A, 13B and 13C in the flowsheet.