the pickling bath 21 and the mg 2 concentration detector 52 measures mg 2 concentration in the pickling bath 21 in a real time manner the ph meter 51 and the mg 2 concentration detector 52 send signals to the control unit 56 according to their measurement the control unit 56 controls operations of the electric control valve 53 the acid metering pump 54 and the water metering pump 55 when a ph value measured by the ph meter 51 is below a predetermined ph threshold gt 7 the control unit 56 receives the signals so as to start the acid metering pump 54 and open the acid in channel 22 to add the pickling bath 21 with acid when a detected value of the mg 2 concentration detector 52 is over a predetermined mg 2 concentration threshold the control unit 56 receives the signals so as to open the electric control valve 53 and open the acid out channel 23 for automatic acid discharge after which the electric control valve 53 is closed while the control unit 56 starts the acid metering pump 54 and the water metering pump 55 to prepare the acid solution again according to a predetermined ratio by regularly measuring the ph value and mg 2 concentration in the pickling bath 21 and automatically adding or changing acid the acid liquid concentration and mg 2 concentration can be maintained in a predetermined range thereby.