A method of separating a liquid mixture selectively separate a substance having a molecular weight of 90 or more from a liquid mixture 31 by a separation membrane. The separation membrane is a MFI type zeolite membrane 2, the liquid mixture 31 is brought into contact with a face on one side of the MFI type zeolite membrane 2, and pressure is reduced on the other side of the MFI type zeolite membrane 2 to trap a membrane-permeable substance 32 permeating the MFI type zeolite membrane 2. The method of separating a liquid mixture can separate a predetermined substance from a liquid mixture without requiring high energy costs.