A process for producing an null-olefin oligomer by oligomerization of an null-olefin in a presence of a Ziegler-based catalyst, the process comprising introducing at least a portion of a reaction product which is taken out of a reactor and has a temperature of 115null C. or higher into an external heat exchanger, cooling the introduced reaction product, maintaining an oligomerization solution taken out of the external heat exchanger at a temperature of 110null C. or higher and returning at least a portion of the oligomerization solution to the reactor. In the production of an null-olefin oligomer, precipitation and adhesion of polymers at the inside of manufacturing apparatuses such as the external heat exchanger and troubles caused thereby can be prevented and a stable continuous operation can be continued for a long time.