There are disclosed a method for separating a liquid mixture in which a permeation performance enhances, and a composition of the liquid mixture containing a hydrocarbon liquid and an alcohol liquid at supply and a composition after membrane permeation are changed by a separating operation, and a device for separating a liquid mixture. The liquid mixture containing the hydrocarbon liquid and the alcohol liquid is used as a supply mixture liquid, and at least part of the supply mixture liquid is, in a liquid state, brought into contact with a membrane supply side of a separation membrane, and discharged in a vapor state through a membrane permeation side of the separation membrane, whereby a composition of a mixture vapor which is balanced with the supply mixture liquid becomes different from a composition of a vapor on the membrane permeation side.