PURPOSE:To perform separation or concentration of a liquid mixture with extremely high efficiency by using a polymer constituted of polystyrene chain having a polysiloxane chain in a phenyl group and polystyrene chain having no polysiloxane chain. CONSTITUTION:A separation membrane is formed of two-component graft polymer comprising a recurrent unit expressed by the formula I and recurrent unit expressed by the formula II. Suitable proportion of the recurrent unit expressed by the formula I is 1-95mol%. Further, preferred mol.wt. of the copolymer is >=10,000, and the preferred film thickness is 0.01-300mum. The membrane is useful for the separation of a liquid mixture, particularly, for the separation of water-alcohol mixture. The separation is performed at -30-150 deg.C and atmospheric pressure - 100atm.