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.