PURPOSE: To disperse efficiently by cooling preliminarily a mixture and feeding the same into an interface lower layer of a tank when two kinds of liquid components are mixed and separating the mixture in which heat is generated for one liquid component of higher specific gravity.

CONSTITUTION: When a mixture in which heat is generated for one liquid component of higher specific gravity by the reaction of self-polymerization or the like is fed into a separation tank 4, oil is separated as an upper layer and diketene and ketene are separated as a lower layer, since the liquid component composed of diketene, ketene and the like is of higher specific gravity. Because of said reason, the mixture is cooled preliminarily and fed into an interface lower layer section. Then, a high temperature area to be formed in the interface lower layer section of the separation tank can be prevented from forming. Separation, therefore, can be carried out without using an inner section cooling tube or the like.