This filter medium for separating a liquid containing water and oil and having a channel for the liquid also has a substrate constituting the channel, and one or more types of nitrogen-containing fluorine-based compounds present on the surface of at least part of the channel. The nitrogen-containing fluorine-based compounds contain, in a molecule thereof, an oil-repellency-imparting group, and one hydrophilicity-imparting group selected from the group consisting of an anionic group, a cationic group, and an amphoteric group.