Ammonia → Ammonium acrylate acrylic acid

In 52.96 g of water, was dissolved 28.32 g of a reagent acrylic acid (guaranteed reagent grade manufactured by Wako Pure Chemical Industries, Ltd.). The resulting solution was cooled in an ice bath and slowly mixed with 18.35 g of a 25% by weight aqueous ammonia solution under stirring while keeping the mixture at a temperature of 30° C. or lower to obtain a 40% by weight aqueous ammonium acrylate/acrylic acid=70/30 solution. The aqueous ammonium acrylate/acrylic acid=70/30 solution in an amount of 90 g was brought to 10° C. and purged with nitrogen for 30 minutes, followed by addition of 0.72 g of ammonium carbonate and 0.147 g of triallylamine. To the resulting solution, were added as an initiator 0.1387 g of ammonium persulfate, 0.0322 g of a 30% by weight aqueous hydrogen peroxide solution, and 0.0016 g of L-ascorbic acid. Polymerization was performed for one hour to obtain a polymer, which was then crushed and dried at 150° C. for one hours in an inert oven with a nitrogen atmosphere. After drying, the polymer was further dried at 170° C. for 30 minutes in an inert oven with a nitrogen atmosphere. The resin obtained as described above was named as a comparative water-absorbing resin (3). The results of measurement of water absorption of the comparative water-absorbing resin (3) are shown in Table 2.