A method of making a group III nitride-based compound semiconductor has the steps of: providing a semiconductor substrate with a polished surface, the semiconductor substrate being of group III nitride-based compound semiconductor; and growing a semiconductor epitaxial growth layer of group III nitride-based compound semiconductor on the semiconductor substrate. The polished surface is an inclined surface that has an off-angle .theta. of 0.15 degrees or more and 0.6 degrees or less to a-face, c-face or m-face of the semiconductor substrate.