A method for producing polyacrylic acid (salt)-based water-absorbing resin includes a step of polymerizing an acrylic acid (salt)-based aqueous monomer solution, the polymerization step involving use of a polymerization reaction apparatus including a polymerization reaction unit covered with a casing, the polymerization step involving a polymerization reaction under internal pressure of the polymerization reaction unit, the internal pressure being slightly reduced and having a pressure reduction level of more than 0 kPa and not more than 10 kPa relative to the ambient pressure at the periphery of the polymerization reaction unit.