The present invention relates to a bis-arylaryloxy catalyst system for the production of ethylene homopolymers or copolymers with α-olefins, which has high catalytic activity. More particularly, it relates to a transition metal catalyst comprising a group-IV transition metal as a central metal, a cyclopentadiene derivative around the central metal, and two aryloxide ligands substituted with aryl derivatives at the ortho-positions, the ligands not being bridged to each other, as well as a catalyst system comprising said catalyst and an aluminoxane co-catalyst or a boron compound co-catalyst, and a method for producing high-molecular-weight ethylene homopolymers or copolymers with α-olefins using the same.