In a process for producing propylene copolymer containing about 80 to 99 mol % propylene and about 1 to 20 mol % ethylene and/or an .alpha.-olefin having 4 to 18 carbon atoms using a Ziegler-Natta catalyst, the improvement which comprises said Ziegler-Natta catalyst comprising:(A) titanium trichloride prepared by reducing titanium tetrachloride with an organoaluminum compound and activating the product; and(B) an organoaluminum compound;with the molar ratio of (B)/(A) being about 20 to 100.