The present invention relates to the method of treating the salt bath liquid. In the surface treatment of the steel material by the use of the high-temperature salt bath mainly comprising sodium hydroxide and sodium nitrate, the salt ingredients contained in the washings generated are separated to be recovered and the metal salts contained are separated in the form of the insoluble salts. The salts contained in the nitrate radical-containing liquid system are recovered as the free acids again, the alkalies being recovered, and the reagents contained in the overflow from the salt-washing tank being recovered. The anode chamber liquid generated in the recoverying operation of the reagents is returned to the washing tank again to increase the concentration of the salts. The anode chamber liquid is poured into the pickling tank to reduce the oxidizing soluble metal salts contained in the washings by iron within the pickling tank, whereby the oxidizing soluble metal salts are insolubilized. The mixture liquid of the overflows from the respective tanks and the washing water for removing the foreign matters in the salt bath is mixed with the nitric acid-containing liquid for pickling the steel material and then sodium hydroxide is supplied to alkalize. Thus, the dissolved metal compounds in the liquids, which have been used for the treatment, are separated into the insoluble solid metal hydroxides and the liquid of the soluble salts without mixing the insoluble alkalies.