PURPOSE: To obtain a paste-type pickling agent which is stable for a long time by adding oxalic acid and water-soluble bridged polyacrylate to a mixture acid liquid of hydrochloric acid and nitric acid.

CONSTITUTION: For example, 6g of water-soluble bridged polyacrylate powder is added to 200g of aq. sol. containing 25% hydrochloric acid, 7% nitric acid and 5% oxalic acid, and then vigorously stirred to prepare a paste-type pickling agent. This pickling agent is in a paste state, stable for more than one month at room temp., and after three months' storage, this paste shows about 10% liquid separation but recovers its viscosity nearly to the initial state of stirred enough. A paste-type pickling agent prepared by adding a nonionic surfactant, sequestering agent, and oxidation inhibitor by 2% each to the above composition is stable for more than three months at room temp.