A method of manufacturing a water-absorbent resin composition containing water-absorbent resin particles and additive particles, the method comprising: a resin particle preparation step of preparing water-absorbent resin particles; and a mixing step of mixing the water-absorbent resin particles which are prepared in the resin particle preparation step, with additive particles in multiple stages by using a plurality of mixing apparatuses and obtaining a water-absorbent resin composition, wherein the mixing step includes a primary mixing stage of carrying out primary mixing of the water-absorbent resin particles with the additive particles by using a first mixing apparatus, a first transport mixing stage of vapor-phase transporting the primary mixture which is obtained in the primary mixing stage, from the first mixing apparatus to the second mixing apparatus through a first transport pipe disposed between the first mixing apparatus and the second mixing apparatus, while mixing the primary mixture in the first transport pipe, a secondary mixing stage of carrying out secondary mixing of a mixture which is obtained in the first transport mixing stage, by using a second mixing apparatus until the additive particles are uniformly dispersed, and a second transport mixing stage of vapor-phase transporting a secondary mixture which is obtained in the secondary mixing stage, from the second mixing apparatus through a second transport pipe which discharges the secondary mixture from the second mixing apparatus, while mixing the secondary mixture in the second transport pipe. a resin particle preparation step of preparing water-absorbent resin particles; and a mixing step of mixing the water-absorbent resin particles which are prepared in the resin particle preparation step, with additive particles in multiple stages by using a plurality of mixing apparatuses and obtaining a water-absorbent resin composition, wherein the mixing step includes a primary mixing stage of carrying out primary mixing of the water-absorbent resin particles with the additive particles by using a first mixing apparatus, a first transport mixing stage of vapor-phase transporting the primary mixture which is obtained in the primary mixing stage, from the first mixing apparatus to the second mixing apparatus through a first transport pipe disposed between the first mixing apparatus and the second mixing apparatus, while mixing the primary mixture in the first transport pipe, a secondary mixing stage of carrying out secondary mixing of a mixture which is obtained in the first transport mixing stage, by using a second mixing apparatus until the additive particles are uniformly dispersed, and a second transport mixing stage of vapor-phase transporting a secondary mixture which is obtained in the secondary mixing stage, from the second mixing apparatus through a second transport pipe which discharges the secondary mixture from the second mixing apparatus, while mixing the secondary mixture in the second transport pipe. a primary mixing stage of carrying out primary mixing of the water-absorbent resin particles with the additive particles by using a first mixing apparatus, a first transport mixing stage of vapor-phase transporting the primary mixture which is obtained in the primary mixing stage, from the first mixing apparatus to the second mixing apparatus through a first transport pipe disposed between the first mixing apparatus and the second mixing apparatus, while mixing the primary mixture in the first transport pipe, a secondary mixing stage of carrying out secondary mixing of a mixture which is obtained in the first transport mixing stage, by using a second mixing apparatus until the additive particles are uniformly dispersed, and a second transport mixing stage of vapor-phase transporting a secondary mixture which is obtained in the secondary mixing stage, from the second mixing apparatus through a second transport pipe which discharges the secondary mixture from the second mixing apparatus, while mixing the secondary mixture in the second transport pipe.