PURPOSE: To maintain a high pickling capacity without increasing an acid concn. at the time of pickling metal at a constant concn. by specifying the ratios of the total ion concn. in a pickling soln. to the ferrous and ferric ion concns.

CONSTITUTION: A hydrofluoric acid-nitric acid mixture 1 with its concn. decreased by pickling is introduced into an acid recovery device 3 from a pipeline 4, water is introduced from a pipeline 6, and the mixture is recovered with an ion-exchange membrane and separated into the recovered hydrofluoric acid and nitric acid and the waste mixture. The recovered acids are sent into a pickling tank 2 through a pipeline 5, and a necessary fresh mixture is supplied to the tank 2 from a pipeline 8 to keep the nitric acid concn. in the pickling soln. 1 at 0.35-0.60N and the hydrofluoric acid concn. at 0.30-0.50N. At this time, the total iron concn. in the soln. 1 is controlled to ≤50g/l, and the concn. ratio of the ferrous ion(Fe2+) to ferric ion(Fe3+) is adjusted to 0.25-2.0. Consequently, constant-concn. pickling is continuously carried out using the pickling soln. having the concentration 1 corresponding to the kind of metal.