PURPOSE: To automatically polish a slitter blade accurately in a short time.

CONSTITUTION: At a thin slitter blade 10, a front blade 10b and a back blade 10c are formed respectively with a cutting angle θ. Supporting bodies 20 and 22 are arranged in the vertical relation at a housing body 18 of a polishing device 4 slidably in the axial direction. Each of the supporting bodies 20 and 22 is arranged on the holding body 18 so that they are inclined by the same angle as the cutting angle θ. A grinding wheel 26 rotatably arranged at the upper supporting body 20 through a supporting shaft is faced in parallel with the front blade 10b of the slitter blade 10. Also, a grinding wheel 28 rotatably arranged at the lower supporting body 22 through a supporting shaft is faced in parallel with the back blade 10c of the slitter blade 10. Both the grinding wheels 26 and 28 are brought into contact with the corresponding blades 10b and 10c and the slitter blade 10 is accurately polished.