As the water-absorbing polymers used in the present invention, there may be mentioned, for example, those obtained by graft-polymerizing a water-soluble polymerizable monomer or a polymerizable monomer which is made water-soluble through hydrolysis such as (meth)acrylic acid or (meth)acrylonitrile, or an oligomer or co-oligomer thereof with a polysaccharide such as a starch or a cellulose, optiona