PURPOSE: To carry out the pickling of a single surface efficiently and economically, by bringing rotatable endless magnetic belts in firm contact with opposite sides of a steel belt or a steel plate over a certain range, and rotating the endless magnetic betls at the same speed as the feeding speed of the steel belt or the steel plate while an acid liquid is fed to the undersurface of the steel belt or the steel plate.

CONSTITUTION: For instance, the rotatable endless magnetic belts 2, 2' that are acid resistant and flexible are mounted firm contact with the opposite sides of the steel belt 1 over a certain range. While the endless magnetic belts 2, 2' are rotated at the same speed as the moving speed of the steel belt 1, an acid liquid is supplied to the undersurface of the steel belt 1 by a spray 7. The acid liquid is prevented by the magnetic belts 2, 2' from adhering to the side surfaces and upper surface of the steel belt 1, so that only the undersurface thereof can be pickled continuously.