To remove the deposited crystallized material of liq. etchant and to prevent the surface of a metallic thin sheet from being roughened by injecting an acidic soln. over the inner face of a splash protective cover.

A metallic thin sheet 10 is conveyed into a cleaning tank 36 from an etching chamber, and high-pressure water is sprayed on one principal plane of the traveling sheet 10 from a spray nozzle 42. Consequently, the liq. etchant depositing on the principal plane is removed, and the peaked part of a resist film is removed at the same time. In this case, a part of the etchant removed from the principal plane of the sheet 10 is deposited on the inner face of a splash preventive cover 44 and dried to form the crystal of iron chloride, etc., and the crystal is gradually accumulated on the inner face of the cover 44. Accordingly, hydrochloric acid is injected against the inner face of the cover 44 from the plural injection ports 34 of a hydrochloric acid feed pipe 26 while the sheet 10 is absent in the cleaning tank 36, and the crystal is dissolved in the hydrochloric acid and washed away from the inner face of the cover 44.