PURPOSE: To maintain the concn. of feed acids constant and rationalize a pickling process by controlling the flow rates of the industrial water and new acids to be added and the concn. of the new acids at the stage of feeding the recovered acids made by concentrating and regenerating the waste acids from a pickling vessel from a storage tank into one feed acid a tank.

CONSTITUTION: The waste soln. (e.g.; hydrochloric acid) of pickling of steel strips or the like in a pickling vessel 1 is fed into a recovery acid installation 2, and the regenerated and recovered acids are once stored 3 and are then fed separately to plural feed acid tanks 4aW4c. The respective detection signals from the densitometer 33 and flowmeter 34 provided to a main pipe 32 are fed to a controller 5, the control signals whereof are fed respectively to flow rate regulating valves 35aW35c. The new acids and industrial water in a new acid tank 7 are fed to the tanks 4aW4c by the flow rate control signal from the controller 5, and are mixed with said regenerated and recovered acids, whereby the acids are prepared to a target concn. The preparation operations are so set sequentially that the operation is started from, for example, the tank 4a and that when the tank 4a goes full, it is shifted to the next tank 4b.