The invention provides a method for producing a group III nitride based semiconductor having a reduced number of crystal defects.A GaN layer 2 is epitaxially grown on a sapphire substrate 1 having C-plane as a main plane (FIG. 1A). Then, the layer is wet-etched by use of a 25% aqueous TMAH solution at 85° C. for one hour, to thereby form an etch pit 4 (FIG. 1B). Then, a GaN layer 5 is grown on the GaN layer 2 through the ELO method (FIG. 1C). The thus-formed GaN layer 5 has a screw dislocation density lower than that of the GaN layer 2.