To restrain the generation of scum in a pickling vessel caused by a brought-in organic material, to prevent the generation of a surface defect on a product steel sheet and to extend the service life of acid by passing a hot-rolled steel strip through a tension-leveler or a temper rolling mill, warm water degreasing equipment and the pickling vessel in order.

This hot-rolled steel strip 1 supplied from a pay-off reel 2 is descaled in a pickling equipment line composed of a tension-leveler or a temper rolling mill 5a, a warm water vessel 6 and a pickling vessel 7 through a shearing and welder 3 and a looper 4 and after tempering it with a rinse vessel 8, a drier 9 and a trimmer 10, this steel strip is wound onto a tension reel 13 through a looper 11 and a shearing machine 12. In this pickling equipment line, a crack is generated on scale generated on the steel strip 1 with hot-rolling by the tension leveler or about 3-5% rolling reduction in the temper rolling mill 5a and rolling oil, etc., in the scale is efficiently degreased in the warm water vessel 6 at ≥ about 75°C. In this way, it is prevented to accumulate by bringing an organic material into the pickling vessel 7 and the generation of scum can be restrained.