To provide a process for making a sheet hydrophilic in a manner in which the initial hydrophilicity of the sheet is excellent and to provide a process for producing a battery separator excellent in the initial hydrophilicity.

The process for making a sheet hydrophilic comprises introducing sulfo groups into the sheet and bringing the sheet into contact with a mixture containing a fluorine gas and an oxygen-containing and/or sulfur-containing reactive gas. The process for producing a battery separator comprises introducing sulfo groups into a sheet containing a fibrous sheet and bringing the sheet into contact with a mixture containing a fluorine gas and an oxygen-containing and/or sulfur-containing reactive gas.