Water absorbent resin production method and usage thereof

A method for producing a water absorbent resin, comprising the steps of:

• (i) polymerizing a monomer including an unsaturated monomer containing a carboxyl group, in the presence of an internal cross-linking agent (A) having two or more radical polymerizable unsaturated groups in its single molecule and in the presence of a non-polymeric internal cross-linking agent (B) having in its single molecule two or more functional groups each of which allows formation of an ester bond or an amide bond by reacting with a carboxyl group so as to obtain a cross-linked polymer hydrogel;
• (ii) crushing the cross-linked polymer hydrogel obtained in the step (i);
• and
• (iii) drying the crushed cross-linked polymer hydrogel products obtained in the step (ii) to obtain the water absorbent resin having an amount of remaining internal cross-linking agent (B) of not more than 1.0×10−5 mol/g, wherein:
• an amount of the internal cross-linking agent (A) is 0.01 mol % or more and 0.2 mol % or less relative to an amount of the unsaturated monomer containing a carboxyl group, and
• a molar ratio (B)/(A) of the internal cross-linking agent (A) and the non-polymeric internal cross-linking agent (B) is 0.01 or more and 1.8 or less.