To provide a novel cross copolymer and resin composition which are improved in heat resistance and compatibility over conventional ethylene/aromatic vinyl compound copolymers and which have low crystallinity, are excellent in softness, transparency and compatibility and show mechanical properties similar to soft vinyl chloride resins, as compared with conventional cross copolymers.A process for producing a cross copolymer, which comprises a coordination polymerization step and a subsequent anionic polymerization step, wherein in the coordination polymerization step, copolymerization of an olefin monomer, an aromatic vinyl compound monomer and an aromatic polyene is carried out by using a single-site coordination polymerization catalyst to synthesize an olefin/aromatic vinyl compound/aromatic polyene copolymer, and then, in the anionic polymerization step, polymerization is carried out in the coexistence of such an olefin/aromatic vinyl compound/aromatic polyene copolymer and an anionic polymerizable vinyl compound monomer by using an anionic polymerization initiator.