PURPOSE: To execute efficient pickling in a short period by passing a strip through a narrow space formed between opposed plates, supplying an acid soln. to the space and oscillating the acid soln. in the space in the direction intersecting orthogonally with the traveling direction of the strip.

CONSTITUTION: The strip 24 is passed through a vertical passing part by means of rollers 22, 23, 28. The vertical passing part is constituted of opposed plates 25, 29 facing the surface of the strip 24. The acid liquid 10 is supplied from nozzles 26, 27, 30, 31 to the space between the strip 24 and the plates 25, 29 to pickle the strip. The falling liquid 10 is collected in a tank 20 and is circulated. ultrasonic oscillators (for example, magnetostrictive oscillators) 32, 35 are respectively provided to the plates 25, 29 to oscillate the plates in the direction perpendicular to the traveling direction of the strip 24, thereby oscillating the liquid 10. The oscillation frequency is made about 1W50kHz. The reactive layer of the scale and acid liquid is thus efficiently destructed.