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 comprising: 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; 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 which includes a tank in an exterior of the treatment container through a supply pipe; a strainer disposed in the supply pipe on an upstream side of the entrance of the spray nozzle; wherein 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, and the strainer includes plural openings each having an opening cross-section smaller than an opening cross-section of the spray exit of the spray nozzle; wherein either condition (a) or condition (b) is met: (a) the apparatus further comprises a cooling device that cools the strainer; (b) the supply pipe for the surface cross-linking agent includes a first pipe that is a header pipe having plural spray nozzles connected thereto and a second pipe that connects the surface cross-linking agent supply source and the first pipe to each other, and the first pipe and the plural spray nozzles are arranged inside the treatment container, a pump is provided on the second pipe, the pump supplying the surface cross-linking agent from the tank that accommodates the surface cross-linking agent through the second pipe, and the strainer is disposed in a portion of the second pipe on an outer side of the treatment container, wherein the second pipe is a pipe path spanning from the exit of the pump to the connection portion to the first pipe that is the header pipe. 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; 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 which includes a tank in an exterior of the treatment container through a supply pipe; a strainer disposed in the supply pipe on an upstream side of the entrance of the spray nozzle; wherein 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, and the strainer includes plural openings each having an opening cross-section smaller than an opening cross-section of the spray exit of the spray nozzle; (a) the apparatus further comprises a cooling device that cools the strainer; (b) the supply pipe for the surface cross-linking agent includes a first pipe that is a header pipe having plural spray nozzles connected thereto and a second pipe that connects the surface cross-linking agent supply source and the first pipe to each other, and the first pipe and the plural spray nozzles are arranged inside the treatment container, a pump is provided on the second pipe, the pump supplying the surface cross-linking agent from the tank that accommodates the surface cross-linking agent through the second pipe, and the strainer is disposed in a portion of the second pipe on an outer side of the treatment container, wherein the second pipe is a pipe path spanning from the exit of the pump to the connection portion to the first pipe that is the header pipe.