To provide a grinding device capable of carrying out grinding work from mill scale grinding to finish grinding to a material to be worked at a lower cost without replacing a grindstone, and also to provide a grinding method.

The grinding device includes a support plate to which the material to be worked is mounted and a rotor in which the grindstone rotatable at a surface almost parallel to the support plate is arranged. The mill scale grinding is carried out while the spindle rotation number of the rotor is reduced, whether or not the mill scale grinding is completed is detected based on the fluctuation of a load current value, and the spindle rotation number of the rotor is increased at a time point when the mill scale grinding is completed.