To achieve high-efficiency laser slicing of SiC, the effects of pulse width on internal modification and cleavage extension were investigated. In the internal modification with an ultrashort pulse laser, high pulse energy enables high-efficiency cleavage extension due to the increase in modification area. However, it was suggested that kerf loss may deteriorate due to multi-stage modification along the laser axis, which is specific to ultrashort pulse lasers. Therefore, we examined the effects of processing using a nanosecond laser to expand the area of cleavage expansion due to thermal effects and suppress the multi-stage modification phenomenon. The results showed that the thermal effect caused expansion of the cleavage at wide laser irradiation intervals and that the multi-stage modification phenomenon did not occur. Furthermore, by controlling the irradiation interval, a processing speed 10 times faster than the conventional process was achieved, and the possibility of highly efficient laser slicing processing was obtained.