To provide a method for manufacturing an ethylene low polymer by using a chromium-based catalyst, the method for recovering a decen, under the condition that a decomposition product of the chromium-based catalyst or the like is hardly produced, from a high boiler in which an ethylene low polymer is separated.

An ethylene low polymer and a high boiler, which contains a chromium-based catalyst, a decen, a tetradecen and a by-product polymer are separated by distillation from a reaction liquid which contains the ethylene low polymer obtained by an ethylene low polymerization reaction using the chromium-based catalyst. Then, the high boiler is concentrated with an evaporation separator 70 and a liquid storage tank 80 so that the concentration of the tetradecen may be at least 5 wt.%, and the decen is separated by evaporation so that the following general formula (1) may be satisfied: /1.2 EXP(850/T)1 (1), wherein T is a temperature of a remaining solution in the evaporation separator 70, and is resident time of the remaining solution in the evaporation separator 70.