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 . <EOS>