An apparatus for producing water-absorbing resin particles for which surface cross-linking treatment is conducted, the surface cross-linking treatment being conducted by spraying a surface cross-linking agent to a water-absorbing resin particle precursor and heating the agent and the precursor, the apparatus includes a treatment container in which the surface cross-linking treatment is conducted, a stirring device including a stirring member disposed in the treatment container, a heating device that heats an inside of the treatment container; and a spray nozzle disposed in the treatment container, the spray nozzle spraying into the treatment container the surface cross-linking agent supplied from a surface cross-linking agent supply source in an exterior of the treatment container through a supply pipe. In a flow path in the spray nozzle spanning from an entrance of the spray nozzle to a spray exit, a point whose opening cross-section is smallest in a flow path through which a fluid passes is the spray exit. A product with further stable physical properties can thereby be acquired.