A method of manufacturing pulp fibers for cellulose nanofiberization from pulp fibers of used hygiene products, comprising steps of: a preparation step of preparing a treatment tank which includes a liquid mixture supply port, and a treatment liquid discharge port and an ozone containing gas supply port that are arranged below the liquid mixture supply port, a liquid mixture supply step of supplying a liquid mixture which includes super absorbent polymers and the pulp fibers that derive from the used hygiene products and water, from the liquid mixture supply port to the treatment tank, an ozone containing gas supply step of supplying ozone containing gas from the ozone containing gas supply port to a treatment liquid in the treatment tank, a pulp fibers for cellulose nanofiberization formation step of forming the pulp fibers for cellulose nanofiberization from the pulp fibers while dissolving at least a portion of the super absorbent polymers in the treatment liquid, by, in the treatment tank, raising the ozone containing gas while lowering the super absorbent polymers and the pulp fibers so as to make the ozone containing gas come into contact with the super absorbent polymers and the pulp fibers, wherein a CT value, which is a product of an ozone concentration in the treatment liquid and an in-tank treatment time, is 100 to 6000 ppm · minute, and a treatment liquid discharge step of discharging the treatment liquid which includes the pulp fibers for cellulose nanofiberization from the treatment liquid discharge port, wherein the pulp fibers for cellulose nanofiberization have a lignin content ratio of 0.1 mass % or less, measured as described in the description. a preparation step of preparing a treatment tank which includes a liquid mixture supply port, and a treatment liquid discharge port and an ozone containing gas supply port that are arranged below the liquid mixture supply port, a liquid mixture supply step of supplying a liquid mixture which includes super absorbent polymers and the pulp fibers that derive from the used hygiene products and water, from the liquid mixture supply port to the treatment tank, an ozone containing gas supply step of supplying ozone containing gas from the ozone containing gas supply port to a treatment liquid in the treatment tank, a pulp fibers for cellulose nanofiberization formation step of forming the pulp fibers for cellulose nanofiberization from the pulp fibers while dissolving at least a portion of the super absorbent polymers in the treatment liquid, by, in the treatment tank, raising the ozone containing gas while lowering the super absorbent polymers and the pulp fibers so as to make the ozone containing gas come into contact with the super absorbent polymers and the pulp fibers, wherein a CT value, which is a product of an ozone concentration in the treatment liquid and an in-tank treatment time, is 100 to 6000 ppm · minute, and a treatment liquid discharge step of discharging the treatment liquid which includes the pulp fibers for cellulose nanofiberization from the treatment liquid discharge port, wherein the pulp fibers for cellulose nanofiberization have a lignin content ratio of 0.1 mass % or less, measured as described in the description.