PURPOSE: Not to flow/scatter water toward the colliding range adjacent to colliding range after high pressure water is collided on the flat surface of metal material and not to scatter the removed scale removed with collision of high pressure water toward the colliding range adjacent to colliding range.

CONSTITUTION: The plural nozzles 5, by which high pressure water 4 divergently injected is collided to the flat surface 3 of a moving metal material 1 in inclined direction as viewing in the width direction of moving route of flat surface and the scale on the flat surface 3 is removed, are juxtaposed along in the width direction of moving route of flat surface, colliding ranges A of high pressure water 4 for the flat surface 3 are spaced for each nozzle 5 apart each other at the state they are overlapped each other as viewing in the longitudinal direction. The injecting direction in all range of the injecting angle B, by which the high pressure water 4 is injected to the flat surface 3 from each nozzle 5, is directed to the direction not to intersect for the colliding range A adjacent to the colliding range by the nozzle 5 in viewing vertical to the flat surface 3.