A method for producing a Group III nitride semiconductor light-emitting device includes forming a first stripe-pattern embossment on the top surface of a sapphire substrate, so that first grooves parallel to the x-axis direction (the c-axis direction of the sapphire substrate) are periodically arranged at specific intervals. Subsequently, an insulating film is formed over the entire surface of the first stripe-pattern embossment. Next, a second stripe-pattern embossment is formed so that second grooves, each having a flat bottom surface, are periodically arranged at specific intervals and parallel to the y-axis direction, which is orthogonal to the x-axis direction. A GaN crystal is grown through MOCVD on side surfaces of each second groove of the sapphire substrate, to thereby form, on the sapphire substrate, an m-plane GaN base layer. An LED device structure is formed on the base layer, to thereby produce a light-emitting device.