To provide an on-line roll grinding method applicable to a work roll in a reverse rolling machine used for a rolled material such as a thick steel plate rolled in a short time, and stably grinding the surface of the work roll with constant depth.

In this on-line grinding method for grinding the work roll during the operation of the rolling machine with a work roll axis runout preventing device, a grinding wheel is pressed to the outer surface of the work roll to grind it when the work roll is rotated in a stand-by position in the state of not biting into the rolled material, and the work roll axis runout preventing device is put in an operating state during grinding to prevent the axis runout of the work roll during grinding.