Abstract: The present invention relates to the techniques of treating a liquid material to be treated, which enables efficient fractional removal of toxic materials such as PCB in a liquid mixture. The treatment method of the invention comprises the step of permitting a liquid material containing materials having different solidifying points to be cooled at a temperature at which desired material(s) is solidified while others are not solidified and the step of separating the solidified material from other liquid materials whereby the desired material is separated, and therefore large facilities are not necessary and toxic materials can be reliably separated and removed.