An object of the present invention is to remove micro-scratches on a surface of a GaN substrate cut from a GaN ingot. The invention is directed to establish a method for surface treatment of a GaN substrate, including heating the surface in an atmosphere containing trimethylgallium, ammonia, and hydrogen. It is preferable that the trimethylgallium feeding rate is 150 μmol/min or higher, the ratio of trimethylgallium feeding rate to ammonia feeding rate (V/III ratio) is 1,200 to 4,000, and the heating temperature is 1,000° C. to 1,250° C. In addition, the temperature of the surface treatment is set to be higher than that of the following GaN growth, and the feed rate of trimethylgallium is lower than that of the growth procedure. RMS of roughness on the substrate was equal to or less than 1.3 nm, and the substrate whose step condition is excellent can be obtained.