The present invention is a method for producing a group III nitride semiconductor layer in which a single crystal group III nitride semiconductor layer (103) is formed on a substrate (101), the method including: a substrate processing step of forming, on the (0001) C-plane of the substrate (101), a plurality of convex parts (12) of surfaces (12c) not parallel to the C-plane, to thereby form, on the substrate, an upper surface (10) that is composed of the convex parts (12) and a flat surface (11) of the C-plane; and an epitaxial step of epitaxially growing the group III nitride semiconductor layer (103) on the upper surface (10), to thereby embed the convex parts (12) in the group III nitride semiconductor layer (103).