A method for producing an oligomer, the method comprising a step of oligomerizing a polymerizable monomer comprising an olefin in the presence of a catalyst comprising an iron complex represented by the following Formula (1) and trialkylaluminum: [In Formula (1), R represents a hydrocarbyl group having 1 to 6 carbon atoms or an aromatic group having 6 to 12 carbon atoms, a plurality of R in the same molecule may be the same or different, R′ represents a free radical having an oxygen atom and/or a nitrogen atom, a plurality of R′ in the same molecule may be the same or different, and Y represents a chlorine atom or a bromine atom].