A method of dividing a brittle substrate allows a cutter head to be easily prepared and ensures enough service life of the cutter head when the cutter head having a top point formed by combinding three surfaces is slided with ridge as a rear side. The cutter head includes a first surface (SD1), a second surface (SD2) and a third surface (SD3). The second surface (SD2) is adjacent to the first surface (SD1) and the third surface (SD3) is adjacent to the second surface (SD2) thus forming the ridge. The third surface (SD3) is adjacent to both of the first surface (SD1) and the second surface (SD2) to form the top point. The ridge is not subjected to chamfering. One one surface (SF1) of the brittle substrate (4), the cutter head (51) slides towards the first surface (SD1) along the ridge (PS), so that a trough line (TL) having a groove-like shape is formed on one surface (SF1) of the brittle substrate (4) without cracks. A crack line (CL) is formed along the trough line (TL), thereby dividing the brittle substrate along the crack line (CL).