To provide a method of manufacturing a separator for battery capable of controlling the bore diameter by general thermal dry, and capable of controlling the air impermeability without substantially changing the physical properties, such as the thickness or surface nature of a porous sheet as a substrate, while using cellulose nanofibers.

A method of manufacturing a separator for battery includes a dispersion liquid preparation step for preparing a dispersion liquid of nanofibers where cellulose nanofibers having a number-average fiber diameter of 1-50 nm are dispersed into a disperse medium, an adhesion step for making the dispersion liquid adhere to a porous sheet having an average bore diameter of 5 μm or less, and a thickness of 50 μm or less, and a dry step for removing the disperse medium by drying the porous sheet to which the dispersion liquid is adhering.