A filter medium (20, 20a) comprising a first porous polytetrafluoroethylene membrane, a first air-permeable support member, a second porous polytetrafluoroethylene membrane, and a second air-permeable support member that are stacked in this order and bonded to one another, wherein a surface of the filter medium (20, 20a) is formed by the first porous polytetrafluoroethylene membrane, a bond strength, as measured by a 180° peel test in accordance with JIS Z 0237, between the first porous polytetrafluoroethylene membrane and the first air-permeable support member is higher than 1.2 N/25 mm and is 2.5 N/25 mm or less, the first air-permeable support member is an embossed non-woven fabric having one or more recessed portions and one or more projecting portions, the embossed non-woven fabric has a sea-island structure when viewed in plan, in the embossed non-woven fabric, a portion corresponding to an island is the projecting portion, where the fibers are not melted, and a portion corresponding to the sea is the recessed portion, where the fibers are melted, or a portion corresponding to an island is the recessed portion, where the fibers are melted, and a portion corresponding to the sea is the projecting portion, where the fibers are not melted, a ratio of the area of the recessed portion or the total area of the recessed portions to the area of the embossed non-woven fabric serving as the first air-permeable support member is more than 15%, and the ratio is determined by using scanning electron microscopy, wherein on the basis of an obtained SEM image of the surface of the embossed non-woven fabric, the proportion of the recessed portion is calculated. a first air-permeable support member, a second porous polytetrafluoroethylene membrane, and a second air-permeable support member that are stacked in this order and bonded to one another, wherein a surface of the filter medium (20, 20a) is formed by the first porous polytetrafluoroethylene membrane, a bond strength, as measured by a 180° peel test in accordance with JIS Z 0237, between the first porous polytetrafluoroethylene membrane and the first air-permeable support member is higher than 1.2 N/25 mm and is 2.5 N/25 mm or less, the first air-permeable support member is an embossed non-woven fabric having one or more recessed portions and one or more projecting portions, the embossed non-woven fabric has a sea-island structure when viewed in plan, in the embossed non-woven fabric, a portion corresponding to an island is the projecting portion, where the fibers are not melted, and a portion corresponding to the sea is the recessed portion, where the fibers are melted, or a portion corresponding to an island is the recessed portion, where the fibers are melted, and a portion corresponding to the sea is the projecting portion, where the fibers are not melted, a ratio of the area of the recessed portion or the total area of the recessed portions to the area of the embossed non-woven fabric serving as the first air-permeable support member is more than 15%, and the ratio is determined by using scanning electron microscopy, wherein on the basis of an obtained SEM image of the surface of the embossed non-woven fabric, the proportion of the recessed portion is calculated.