A cyclic olefin-based copolymer of the first invention of the application includes a constituent unit (A) derived from an α-olefin having 2 to 20 carbon atoms, a constituent unit (B) derived from a cyclic olefin having no aromatic ring, and a constituent unit (C) derived from a cyclic olefin having an aromatic ring. A medical container of the second invention of the application includes a cyclic olefin-based copolymer, which includes a constituent unit (A) derived from an α-olefin having 2 to 20 carbon atoms, a constituent unit (B) derived from a cyclic olefin having no aromatic ring, and a constituent unit (C) derived from a cyclic olefin having an aromatic ring.本案第一發明的環狀烯烴系共聚物具有:自碳原子數為2~20的α-烯烴導出的構成單元(A);自不具有芳香環的環狀烯烴導出的構成單元(B);以及自具有芳香環的環狀烯烴導出的構成單元(C)。本案第二發明的醫療用容器包含環狀烯烴系共聚物,所述環狀烯烴系共聚物具有:源自碳原子數為2~20的α-烯烴的構成單元(A);自不具有芳香環的環狀烯烴導出的構成單元(B);以及自具有芳香環的環狀烯烴導出的構成單元(C)。