NOVELTY - Production of group III nitride semiconductor light emitting device comprises laminating a buffer layer (12) comprising group III nitride compound on a substrate; and laminating n-type semiconductor layer (14) provided with an underlying layer (14a), light emitting layer (15), and p-type semiconductor layer (16) sequentially, on the buffer layer. It comprises forming the buffer layer having a composition of aluminum gallium nitride by activating with plasma; reacting metallic gallium source and gas containing a group V element; and forming the underlying layer on the buffer layer.    USE - Method of producing group III nitride semiconductor light emitting device for use in lamp (claimed). Uses include but are not limited to LED, laser diode, electronic device such as heterojunction bipolar transistors and high electron mobility transistors, and photoelectric conversion devices such as solar cells and light receiving components.    ADVANTAGE - The method provides group III nitride semiconductor light emitting device with excellent light emitting properties and with excellent productivity. The buffer layer can be formed as a layer with favorable orientation; and an underlying layer with a favorable level of crystallinity can be formed on the buffer layer. Thus, no lattice mismatch occurs between the substrate and a semiconductor layer comprising the group III nitride semiconductor. By forming the buffer layer so as to cover each of the substrate planes, it is possible to prevent the substrate from reacting with the source gas. The time required for the film forming process can be shortened leading to improved productivity.    DETAILED DESCRIPTION - Production of group III nitride semiconductor light emitting device comprises laminating a buffer layer (12) comprising group III nitride compound on a substrate; and laminating n-type semiconductor layer (14) provided with an underlying layer (14a), light emitting layer (15), and p-type semiconductor layer (16) sequentially, on the buffer layer. It comprises forming the buffer layer having a composition of aluminum gallium nitride (AlxGa1-xN) by activating with plasma; reacting metallic gallium source and gas containing a group V element; and forming the underlying layer on the buffer layer.    x= greater than or equal to 0, less than 1.    An INDEPENDENT CLAIM is included for a group III nitride semiconductor light emitting device comprising buffer layer including group III nitride compound laminated on substrate; and n-type semiconductor layer provided with underlying layer, light emitting layer, and p-type semiconductor layer sequentially laminated on the buffer layer.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic cross-sectional view of a group III nitride semiconductor light emitting device.    Buffer layer (12)    N-type semiconductor layer (14)    Underlying layer (14a)    Light emitting layer (15)    P-type semiconductor layer (16)