To provide a continuous polishing control method of a steel plate in coil which can maintain a polished grain of the steel plate in coil uniformly in the longitudinal direction irrespective of wear of a polishing belt.

When surface polishing is done by bringing a polishing belt 1 which is rotated and driven by a contact roll 3 into contact with a surface of a steel plate in coil W and applying polishing pressure on the steel plate in coil W by a billy roll 4 from a rear surface side, changes of a polished grain on a polishing surface on an out side of a polishing device 10 are detected by a polishing grain measurement sensor 11, and at least one of a plurality of polishing conditions which affect the polished grain is automatically controlled in the continuous polishing device 10 by a polishing control device 20 based on the detected value to make the polished grain in the longitudinal direction of the steel plate in coil W uniform. As the polishing conditions, steel plate in coil feed speed, polishing belt peripheral speed, polishing current, breaker angle, offset among of contact roll and billy roll, or the like can be used.