To provide a surface treatment method and a surface treating apparatus capable of performing at high speed and with improved flatness, and can flatten the surface of a nitride compound semiconductor.

The surface treating apparatus comprises an electrolyte supply port 15 for supplying an KOH electrolyte 14 containing a metal fine particle and an abrasive material; a retainer 40 that has an opening at the upper portion and retains the KOH electrolyte 14 supplied from the electrolyte supply port 15; a wafer holder 12 that fixes a GaN substrate 11 and dips the surface of the GaN substrate 11 into the KOH electrolyte 14 in the retainer 40 from the upper portion of the retainer 40, so that it comes into contact with the KOH electrolyte 14; a load 13 installed on the wafer holder 12; an apparatus casing 16; a polishing pad 17 for polishing the surface of the GaN substrate 11; and an ultraviolet light source 42.