The objective of the present invention is to provide a composite separation membrane which is excellent in not only a liquid permeable performance and a separation performance relatively but also a durability and which is particularly useful as a membrane for liquid treatment, and a method for treating a liquid by using the composite separation membrane. The composite separation membrane according to the present invention is characterized in comprising a supporting base material and a complex layer, wherein the complex layer is placed on the supporting base material, the complex layer comprises oxidized metal nanosheets, graphene oxide and an alkanolamine, and at least one of the alkanolamine is present between the oxidized metal nanosheets.