A polishing method can process and flatten, in a practical processing time and with high surface accuracy, a surface of a substrate of a Ga element-containing compound semiconductor. The polishing method includes: bringing a Ga element-containing compound semiconductor substrate (16) into contact with a polishing tool (10) in the presence of a processing solution (14) comprising a neutral pH buffer solution containing Ga ions; irradiating a surface of the substrate with light or applying a bias potential to the substrate, or applying a bias potential to the substrate while irradiating the surface of the substrate with light, thereby forming Ga oxide (16a) on the surface of the substrate; and simultaneously moving the substrate and the polishing tool relative to each other to polish and remove the Ga oxide formed on the surface of the substrate.