Abstract: A method for producing a water absorbent resin by polymerizing a water-soluble monomer having a polymerizable unsaturated group (A) and a crosslinking agent (B) as the essential components by radical polymerization in the presence of water or, polymerizing a water-soluble monomer having an unsaturated group (A) and a crosslinking agent (B) by radical-graft copolymerization with a water-soluble polymer (C) as the backbone polymer in the presence of water, wherein a water-based solid material capable of being endothermally fused or dissolved into water (D) is added when initiating the polymerization, with at least a part of the water-based solid material (D) in a solid state. By the method of the present invention, a water absorbent resin having a high molecular weight and a high absorption capacity with little water-soluble component can be obtained with ease.