Disclosed herein is a water treatment process for efficiently removing oils from oil-containing water. The water treatment process is a method for treating oil-containing water, comprising the steps of: preparing an oil-adsorbing material that is made of a lipophilic resin such as a pyridine resin and that has many pores in its surface and bears hydrophilic groups on an inner surface of the pores; and bringing oil-containing water into contact with the surface of the oil-adsorbing material. The step of preparing an oil-adsorbing material includes the step of converting some of hydrophobic groups, such as nitrogen-containing aromatic rings, on the inner surface of the pores to hydrophilic groups such as quaternized amine or sulfonic acid.
