PURPOSE: To grind a rolling roll on line over a long period of time and in excellent grinding efficiency, by pressing a non-conductor made of ultra-hard material, solidly stuck to the tip of an electrode whetstone, to the rolling roll, and supplying DC current while feeding an electrolyte to a place between the electrode and the roll.

CONSTITUTION: In a device for on-line grinding a rolling roll 6 by setting an electrode whetstone 9 to a cathode and the rolling roll 6 to an anode, and feeding an electrolyte 22 to a place between both electrodes; a nonconductor made of ultra-hard material, in which abrasive grains are embedded if necessary, is solidly stuck to the tip of said whetstone 9, and the whetstone 9 is pressed to the roll 6 through said non-conductor by adding a hydraulic pressure 26 to a whetstone pressing device 8 from a pipe 11. Said pressing pressure is set to the degree of 0.5W6kg/cm2, and the distance between the whetstone 9 and the roll 6 is maintained within the range of 0.2W0.4mm. Further, a sticking of spattered electrolyte 22 to a material 5 to be rolled is prevented, and at the same time, the electrolyte is enabled to circulate by storing it, by providing holding plates 12, 13.