To provide a method of producing propylene type polymers having improved processability without loss of good mechanical properties of polypropylene.

The method is based on copolymerization of a polyene compound with propylene to obtain an isopropylene-based polymer. The method comprises copolymerizing a polyene compound with propylene in the presence of a catalyst prepared by containing together (A) modified particles obtained by contacting (a) a compound of formula M1L1m (wherein, M1 is a typical metal atom of the group 1, 2, 12, 14 or 15; and L1's are each a hydrogen atom, a halogen atom or a hydrocarbon group), (b) a compound of formula R1t-1TH (wherein, R1 is an electron-attracting group or a group containing an electron- attracting group; and T is an atom of the group 15 or 16) and (c) a compound of formula R2t-2TH2 (wherein, R2 is a hydrocarbon group or a halogenated hydrocarbon group) with (d) base particles, (B) a transition metal compound and an (C) organoaluminum compound.