Disclosed is transition metal complex that serves as a catalytic component with which 1-hexene can be produced efficiently with excellent selectivity, even under high temperature conditions, by means of an ethylene trimerization redaction. Also disclosed is a method for economically preparing a butyl-branched ethylene polymer, even under high temperature conditions, by using said transition metal complex as an ethylene trimerization catalyst, and polymerizing ethylene in the presence of an olefin polymerization catalyst that is obtained by bringing an olefin copolymerization catalyst and an activating co-catalytic component into contact with one another. Said transition metal complex is represented by the following general formula (1), wherein M 1 represents a Group 4 transition metal atom, and R 1 through R 11 and X 1 through X 3 each independently represent a hydrogen atom, a halogen atom, or a specific organic group.