PURPOSE: To make it possible to uniformly polish even thin print boards without damage in a polishing device for print boards or the like, by providing a warp correcting plate which moves obliquely up and down between a guide plate and a polishing mechanism comprising a polishing brush, etc.

CONSTITUTION: A thin plate 9 which is transferred by a travelling conveyor 3, are corrected in warp at its end by pinch rollers 4, and is conveyed through a space between a polishing brush 2a and a back-up roller 14 by being guided widthwise by a guide plate 10. When a detector 11 detects the pass of the plate 9, a warp correcting plate 13 which is adapted to obliquely move up and down, moves down to a position defining a gap suitable for the thickness of the thin plate 9, and corrects the warp of the thin plate 9 at its leading end just before being make into contact with the polishing brush 2, so that the thin plate 9 is smoothly inserted. Further, the detection signal is delayed in accordance with the distance by means of a timer 12, that is, the polishing brush 2 is reversely rotated by the delayed signal in a position where the leading end of the thin plate 9 is clamped by the rear pinch rollers 4, and then is normally rotated after the rear end of the plate 9 has passed the polishing brush 2. Thus, even a very thin print board may be uniformly polished without damage.