A filter medium for an air filter collecting dust in an air flow has a principal collection layer including a porous polytetrafluoroethylene film. A surface of the filter medium is provided with a plurality of embossed projections. Preferably, a processed filter medium maintains a zigzag shape by pleating an unprocessed filter medium to create mountain folds and valley folds. Preferably, an air filter unit includes such a processed filter medium and a frame body holding the processed filter medium.