A method of producing pulp fibers for cellulose nanofiberization from pulp fibers contained in a used sanitary product, the method comprising: a preparation step of preparing a treatment tank including a pulp fibers-containing material supply port, a treatment liquid discharge port, and an ozone-containing gas supply port arranged at a lower position of the treatment tank; a pulp fibers-containing material supply step of supplying pulp fibers-containing material to the treatment tank from the pulp fibers-containing material supply port; an ozone-containing gas supply step of supplying ozone-containing gas to a treatment liquid in the treatment tank from the ozone-containing gas supply port; a pulp fibers for cellulose nanofiberization forming step of forming the pulp fibers for cellulose nanofiberization having a lignin content ratio of 0.1 mass % or less, measured according to the method described in the description, from the pulp fibers, by supplying the ozone-containing gas to the treatment tank containing the treatment liquid containing the pulp fibers-containing material containing superabsorbent polymers and the pulp fibers which derive from the used sanitary product, as well as bringing the pulp fibers-containing material into contact with the ozone-containing gas while raising the ozone-containing gas in the treatment tank, to dissolve at least a part of the superabsorbent polymers in the treatment liquid; and a treatment liquid discharge step of discharging the treatment liquid containing the pulp fibers for cellulose nanofiberization from the treatment liquid discharge port, characterized in that the treatment liquid discharge port is arranged on an upper side of the pulp fibers-containing material supply port, and in the pulp fibers for cellulose nanofiberization forming step, the pulp fibers-containing material is brought into contact with the ozone-containing gas while raising the pulp fibers-containing material. a preparation step of preparing a treatment tank including a pulp fibers-containing material supply port, a treatment liquid discharge port, and an ozone-containing gas supply port arranged at a lower position of the treatment tank; a pulp fibers-containing material supply step of supplying pulp fibers-containing material to the treatment tank from the pulp fibers-containing material supply port; an ozone-containing gas supply step of supplying ozone-containing gas to a treatment liquid in the treatment tank from the ozone-containing gas supply port; a pulp fibers for cellulose nanofiberization forming step of forming the pulp fibers for cellulose nanofiberization having a lignin content ratio of 0.1 mass % or less, measured according to the method described in the description, from the pulp fibers, by supplying the ozone-containing gas to the treatment tank containing the treatment liquid containing the pulp fibers-containing material containing superabsorbent polymers and the pulp fibers which derive from the used sanitary product, as well as bringing the pulp fibers-containing material into contact with the ozone-containing gas while raising the ozone-containing gas in the treatment tank, to dissolve at least a part of the superabsorbent polymers in the treatment liquid; and a treatment liquid discharge step of discharging the treatment liquid containing the pulp fibers for cellulose nanofiberization from the treatment liquid discharge port, characterized in that the treatment liquid discharge port is arranged on an upper side of the pulp fibers-containing material supply port, and in the pulp fibers for cellulose nanofiberization forming step, the pulp fibers-containing material is brought into contact with the ozone-containing gas while raising the pulp fibers-containing material.