In a polishing method of a GaN substrate according to this invention, first, while supplying a polishing solution 27 containing abrasives 23 and a lubricant 25, onto a platen 101, the GaN substrate is polished using the platen 101 and the polishing solution 27 (first polishing step). Then the GaN substrate is polished using the platen 101 in which abrasives 29 are buried, while supplying a lubricant 31 onto the platen 101 in which the abrasives 29 are buried (second polishing step).