The nonwoven fabric used as a surface fiber layer preferably includes spun-bond nonwoven fabric, heat roll nonwoven fabric, suction heat-bond nonwoven fabric, and so forth which are made of synthetic fibers, such as polyethylene, polypropylene, polyester, polyurethane, and composites thereof, having been rendered hydrophilic by hydrophilic treatment and having a diameter of 10 to 100 ??m.