To provide a method of producing a propylene-olefin copolymer with a high polymerization activity while removing a polymerization inhibitor in the propylene and the olefin by a simple method when carrying out the copolymerization of propylene and can olefin by using a metallocene catalyst system.

The propylene and the olefin treated with an activated alumina and a molecular sieve in order, or with the molecular sieve, the activated alumina and the molecular sieve in order are copolymerized in the presence of a metallocene catalyst system to provide the propylene-olefin copolymer having ≤50 wt.% olefin content. In other aspect, the propylene and the olefin, treated with the activated alumina and a molecular sieve in order, or with the molecular sieve, the activated alumina and the molecular sieve in order are copolymerized in the presence of the metallocene catalyst system to provide the propylene- olefin copolymer having ≤50 wt.% olefin content.