In an apparatus for producing the water-absorbent resin, gas heated by the polymerization heat accompanied by a polymerization reaction in a reactor main body flows through a first pipe into a heat exchanging structure, and a cooled fluid obtained by the cooling flows through a second pipe into the reactor main body. In the apparatus, a first pipe temperature adjusting portion adjusts a temperature of the first pipe to fall within a predetermined temperature range, and a polymerization reactor temperature adjusting portion adjusts a temperature of an upper portion of the reactor main body to fall within a predetermined temperature range.