Abstract: [Problem] To provide a membrane for the forward osmosis method, which keeps a high porosity, reduces concentration polarization by appropriately controlling the pore distribution, achieves both high water permeability and a self-supporting property, and has high chemical durability such that are membrane is applicable to various draw solutions. [Solution] A separation membrane having a structure inclined from an outer surface side to an inner surface side, a ratio between a thickness of a dense layer having a dense polymer density and a thickness of a coarse layer having a coarse polymer density being in a range of 0.25?(the thickness of the coarse layer)/[(the thickness of the dense layer)+(the thickness of the coarse layer)]?0.6, when measuring polymer density distribution in a thickness direction of the separation membrane by Raman spectroscopy.

Abstract: [Problem] To provide a membrane for the forward osmosis method, which keeps a high porosity, reduces concentration polarization by appropriately controlling the pore distribution, achieves both high water permeability and a self-supporting property, and has high chemical durability such that the membrane is applicable to various draw solutions. [Solution] A separation membrane having a structure inclined from an outer surface side to an inner surface side, a ratio between a thickness of a dense layer having a dense polymer density and a thickness of a coarse layer having a coarse polymer density being in a range of 0.25?(the thickness of the coarse layer)/[(the thickness of the dense layer)+(the thickness of the coarse layer)]?0.6, when measuring polymer density distribution in a thickness direction of the separation membrane by Raman spectroscopy.