That is, the water-containing gel-form cross-linked polymer is dispersed in an organic solvent of a hydrocarbon such as hexane and the like, and azeotropically dried so that the water-containing gel-form cross-linked polymer has a moisture content of 40 wt % or less (lower limit is 0 wt %, preferably, 5 wt %), and preferably 30 wt % or less.