The present invention relates to a method and device for manufacturing an alpha-olefin oligomer by low polymerization reaction of an alpha-olefin in a liquid phase part in a reactor in the presence of a catalyst, and more particularly relates to a method and device whereby generation of a polymer on a tube plate surface in a top part of a shell-and-tube heat exchanger used for heat removal is suppressed, and continuous operation is performed stably for a long time when low-polymerization reacting an alpha-olefin such as ethylene and manufacturing an alpha-olefin oligomer such as 1-hexene.