PURPOSE: To prepare the titled block copolymer with a specific content of ethylene having balanced impact resistance and rigidity, by polymerizing propylene or copolymerizing the propylene with ethylene, and copolymerizing the propylene with ethylene at a specific weight ratio.

CONSTITUTION: Propylene alone is polymerized or copolymerized with a small amount of ethylene, and then propylene is copolymerized with ethylene at ≥(20/ 80) weight ratio between the propylene and ethylene, e.g. by the mass polymerization method, etc. to give the aimed propylene-ethylene block copolymer containing segments with ≥80wt% content of ethylene.