Provided is a method of producing a water-absorbent resin comprising a step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of an internal-crosslinking agent, and a step of performing post-crosslinking, wherein: the step of polymerizing comprises one or more polymerization steps; all the polymerization steps are performed in the presence of both one or more azo based compounds and peroxides so that the both are coexistent in the all steps; in each polymerization step, a proportion of the azo compounds is 40-95 mass % relative to an amount of the azo compounds and the peroxides; the water-absorbent resin has a water-absorption capacity of physiological saline under a load of 4.14 kPa at 120 minutes passed from the start of water absorption of 20 ml/g or more; and the degree of swelling under a load at 30 minutes is 70%.