PURPOSE: To keep a water spray nozzle so as to direct in the tangential direction to the grinding area of a strip in conformity with the vertically following displacement of brush rolls in a brush roll machine to grind the upper and lower surfaces of a metallic strip pass line with brush rolls arranged vertically and facing each other and displaced vertically following the vertical motion of the line.

CONSTITUTION: When a water spray nozzle 2 can change direction vertically and brush rolls 1 on the upper and lower surfaces of a metallic strip pass line P are displaced vertically following the vertical motion of this line, by providing a detector to detect the displacement, a driving machine 4 to change the direction of a nozzle 2, the detector 11 to detect its mechanical momentum and a controller to which these detectors are connected, the nozzle is changed with its direction so that it directs in the tangential direction to the polishing area of a strip.