Abstract: The present invention relates to a mixed liquid separating apparatus for separating a particular substance from a mixed liquid containing at least two kinds of substances C and O of which follow-rotating property is different. It comprises a cylindrical outer member 1, a rod-shaped inner member 2 and a driving means 3. The outer member 1 has an inducing port 11 for inducing the mixed liquid at one end thereof, and an expelling port 16 for expelling the separated particular substance O at other end thereof. The inner member 2 is disposed coaxially with said outer member 1 to be relatively rotatable thereto. The driving means 3 relatively rotates said outer member 1 and said inner member 2. Here, at least one of an inner peripheral surface of said outer member 1 and an outer peripheral surface of said inner member 2 has a spiral guiding wall for guiding the particular substance O from the one end to the other end, by the relative rotation between said outer member and said inner member.