The present invention relates to a method for production of a novel water-absorbent resin and a method for production of a novel water-retaining material using the same. Conventional water-retaining materials are classified into organic water-retaining materials which use a fibrous material as a main component and are impregnated with a polymer solution and inorganic water-retaining materials in which a powder of e.g. silica gel or pulp, etc. is kneaded with a binder and formed into the shape of an article. The former organic water-retaining material is produced by the steps of impregnating a fibrous material with a polymer solution, draining the solvent, followed by drying and heat treatment, or the steps of impregnating a fibrous material with a polymer solution, subsequently, heat-treating the material to deactivate bacteria or the like, then forming it into the shape of an article and drying the same at a temperature of 100.degree. C. or higher, whereas the latter inorganic water-retaining material is produced by kneading and forming a powder of a fibrous material and a binder. It is possible to obtain water-absorbent resin having such a high absorbency as is comparable with the inorganic water-retaining material by the aforesaid production process but the product is poor in strength and is limited in shape. Further, since the organic water-retaining material is formed into the shape of a paper-like sheet by the steps of forming, stretching and heat-treating, it is impossible to make the shape fine. If it is necessary to dry a fibrous material for the purpose of increasing the absorbency of the water-absorbent resin, a binder is used so that the amount of the binder in the water-absorbent resin increases. As a method for production of a water-absorbent resin which does not employ a binder, a method using a solution of an ultra-high-molecular-weight polyethylene having a number average molecular weight (Mn) of 1.5.times.10.sup.6 or higher is known. In general, it is a general method of producing a water-absorbent resin by sintering a starting material after removing it from the solution. In this method, a water-absorbent resin in a gel state, in which a fibrous material, etc. is impregnated with the ultra-high-molecular-weight polyethylene having a number average molecular weight (Mn) of 1.5.times.10.sup.6 or higher is obtained. Since this product is in a gel state and is in the form of a sheet, this method has the advantages that it is possible to make a thick article without limitation of the thickness and that the product can be made in a complicated shape. However, since the water-absorbent resin obtained by this method is in the gel state, its absorbency is insufficient. Therefore, if a number of the water-absorbent resins are used together to impart a high absorbency to the article as a whole, these resins will become entangled with each other or even though they are entangled with each other, the absorbency will not be increased so much. Further, when the gel product is heated, the starting material will be deformed due to its melting and will stick to an inner wall of the heating vessel to result in the occurrence of a defect and cause the inner wall of the vessel to be damaged. A method for production of a water-absorbent resin having improved absorbency has been proposed in which ultra-high-molecular-weight polyethylene is dissolved in a small amount of an organic solvent and is subjected to casting followed by drying to remove the organic solvent (JP-A-61-60608). The water-absorbent resin obtained by this method is porous so that the number of the water-absorbent resins which can be held together in one place is small. In case of using this resin for a pad for absorption and retaining a liquid, it will easily drop apart from the pad by the pressure or weight of the liquid and will leak out to the outside so that a large number of the water-absorbent resin has to be used to obtain a large amount of the absorbent resin and consequently, a larger pad and a more amount of the solvent are required, thus creating a problem in economy. Further, since the absorbent resin is produced by casting, the amount of the resin which can be produced is limited by the amount of the solvent. Also, a method for production of a water-absorbent resin in which a fibrous material is impregnated with ultra-high-molecular-weight polyethylene in a specific solvent and the material is thereafter subjected to extraction by supercritical carbon dioxide to obtain a water-absorbent resin has been proposed (JP-A-3-239513). This water-absorbent resin obtained by this method is not porous so that it can hold a liquid. Therefore, the water-absorbent resin of a large size can be produced but the method is poor in productivity and is expensive in cost. Further, in general, when a hydrophobic fiber is impregnated with ultra-high-molecular-weight polyethylene, the fiber is swelled. When a hydrophilic fiber, such as pulp is used, a great pressure must be applied to impregnate the fiber because of poor affinity of the fiber with ultra-high-molecular-weight polyethylene. Since such an application of great pressure increases the cost of production, the method is not economical. Also, water-absorbent resin obtained by incorporating hydrophilic pulp into ultra-high-molecular-weight polyethylene is in the gel state and will absorb only a small amount of a liquid. Further, when a specific solvent which is capable of swelling the hydrophobic fiber or the ultra-high-molecular-weight polyethylene is used, it is necessary to remove the solvent by extraction or the like after impregnation of the fiber with the specific solvent. If the specific solvent is not removed completely, the water-absorbent resin will be in the gel state. Therefore, a certain time is necessary to completely remove the solvent. Further, a water-absorbent resin obtained by mixing and molding fine particles of ultra-high-molecular-weight polyethylene into a water-absorbent resin composed of polyvinyl alcohol and sodium sulfate is inferior in uniformity of the obtained water-absorbent resin to a water-absorbent resin obtained by molding the starting material at a temperature not lower than 100.degree. C. and higher than the melting point of the ultra-high-molecular-weight polyethylene. In a water-absorbent resin obtained by kneading an aqueous solution of ultra-high-molecular-weight polyethylene and water in a kneading machine, it is impossible to obtain a uniform mixture of the water-absorbent resin and the aqueous solution of the ultra-high-molecular-weight polyethylene.