A composite membrane formed by laminating a layer of a moisture-permeable resin on one surface of a hydrophobic porous membrane, the layer of the moisture-permeable resin being included in a reinforcing porous membrane and extending out of an upper surface of the reinforcing porous membrane, wherein the moisture permeable resin is selected from the group comprising a polystyrene sulfonic acid, a polyvinyl alcohol, a polyurethane, vinyl alcohol copolymers, fluorine-based ion exchange resins, resins having protic hydrophilic groups in the repeating units thereof and resins having aprotic hydrophilic groups in the repeating units thereof, wherein the hydrophobic porous membrane and the reinforcing porous membrane are independently selected from a membrane made from a resin selected from the group comprising a polyolefin, a polycarbonate, a polystyrene, a polyvinyl chloride, a polyvinylidene chloride, a polyester and a fluorocarbon polymer, wherein the upper surface of the reinforcing porous membrane is the surface of the reinforcing membrane opposite the hydrophobic porous membrane and the lower surface of the reinforcing porous membrane is the surface of the reinforcing porous membrane facing the hydrophobic porous membrane, the composite membrane being characterized in that the layer of the moisture permeable resin extends out of a lower surface of the reinforcing porous membrane to form an exposed portion of the layer of moisture permeable resin which is in contact with the reinforcing porous membrane and the hydrophobic porous membrane and at least a part of the layer of the moisture-permeable resin that extends out of the lower surface of the reinforcing porous membrane is included into the hydrophobic porous membrane but does not extend out of a lower surface of the hydrophobic porous membrane. wherein the moisture permeable resin is selected from the group comprising a polystyrene sulfonic acid, a polyvinyl alcohol, a polyurethane, vinyl alcohol copolymers, fluorine-based ion exchange resins, resins having protic hydrophilic groups in the repeating units thereof and resins having aprotic hydrophilic groups in the repeating units thereof, wherein the hydrophobic porous membrane and the reinforcing porous membrane are independently selected from a membrane made from a resin selected from the group comprising a polyolefin, a polycarbonate, a polystyrene, a polyvinyl chloride, a polyvinylidene chloride, a polyester and a fluorocarbon polymer, wherein the upper surface of the reinforcing porous membrane is the surface of the reinforcing membrane opposite the hydrophobic porous membrane and the lower surface of the reinforcing porous membrane is the surface of the reinforcing porous membrane facing the hydrophobic porous membrane, the composite membrane being characterized in that the layer of the moisture permeable resin extends out of a lower surface of the reinforcing porous membrane to form an exposed portion of the layer of moisture permeable resin which is in contact with the reinforcing porous membrane and the hydrophobic porous membrane and at least a part of the layer of the moisture-permeable resin that extends out of the lower surface of the reinforcing porous membrane is included into the hydrophobic porous membrane but does not extend out of a lower surface of the hydrophobic porous membrane.