A method for mixing a water-absorbent resin and a surface cross-linking agent-containing solution to repress aggregates is equipped. The water-absorbent resin and the surface cross-linking agent-containing solution is mixed with a plow-shaped mixing device which is equipped with an inner wall formed of material manifesting an interfacial contact angle of not less than 60.degree. to water and possessing a temperature of heat distortion of not lower than 70.degree. C., a plow-shaped stirring vane and a disintegrating blade.