A filter medium includes a first porous polytetrafluoroethylene membrane, a first air-permeable support member, a second porous polytetrafluoroethylene membrane, and a second air-permeable support member. A surface of the filter medium is formed by the first porous polytetrafluoroethylene membrane. A bond strength, as measured by a 180° peel test, between the first porous polytetrafluoroethylene membrane and the first air-permeable support member is higher than 1.2 N/25 mm and higher than a bond strength, as measured by the 180° peel test, between the first air-permeable support member and the second porous polytetrafluoroethylene membrane.
