An acid solution preparation device for preparing an acid solution suitable for being used for pickling of a steel plate , the device comprising: a sealed tank for storing the acid solution ; a gas supply part for supplying an oxygen-containing gas from outside of the sealed tank to the sealed tank ; and a purge part for discharging a gas in the sealed tank to the outside, wherein the purge part includes: a purge pipe through which the gas discharged from the sealed tank flows; and a second valve disposed on the purge pipe , for regulating a flow rate of the gas discharged from the sealed tank via the purge pipe , characterized in that the acid solution preparation device further comprises: a concentration sensor for measuring an oxygen concentration in gas phase in the sealed tank ; a pressure sensor for measuring a pressure in the sealed tank ; and a controller connected to both the concentration sensor and the pressure sensor , wherein the controller is configured to adjust the opening degree of the second valve such that an opening degree thereof is adjusted • based on measurement result by the concentration sensor or • based on a measurement result by the pressure sensor and the concentration sensor . a sealed tank for storing the acid solution ; a gas supply part for supplying an oxygen-containing gas from outside of the sealed tank to the sealed tank ; and a purge part for discharging a gas in the sealed tank to the outside, wherein the purge part includes: a purge pipe through which the gas discharged from the sealed tank flows; and a second valve disposed on the purge pipe , for regulating a flow rate of the gas discharged from the sealed tank via the purge pipe , characterized in that the acid solution preparation device further comprises: a concentration sensor for measuring an oxygen concentration in gas phase in the sealed tank ; a pressure sensor for measuring a pressure in the sealed tank ; and a controller connected to both the concentration sensor and the pressure sensor , wherein the controller is configured to adjust the opening degree of the second valve such that an opening degree thereof is adjusted • based on measurement result by the concentration sensor or • based on a measurement result by the pressure sensor and the concentration sensor . a purge pipe through which the gas discharged from the sealed tank flows; and a second valve disposed on the purge pipe , for regulating a flow rate of the gas discharged from the sealed tank via the purge pipe , characterized in that the acid solution preparation device further comprises: a concentration sensor for measuring an oxygen concentration in gas phase in the sealed tank ; a pressure sensor for measuring a pressure in the sealed tank ; and a controller connected to both the concentration sensor and the pressure sensor , wherein the controller is configured to adjust the opening degree of the second valve such that an opening degree thereof is adjusted • based on measurement result by the concentration sensor or • based on a measurement result by the pressure sensor and the concentration sensor . a concentration sensor for measuring an oxygen concentration in gas phase in the sealed tank ; a pressure sensor for measuring a pressure in the sealed tank ; and a controller connected to both the concentration sensor and the pressure sensor , wherein the controller is configured to adjust the opening degree of the second valve such that an opening degree thereof is adjusted • based on measurement result by the concentration sensor or • based on a measurement result by the pressure sensor and the concentration sensor . • based on measurement result by the concentration sensor or • based on a measurement result by the pressure sensor and the concentration sensor . <EOS>