A characteristic feature of the invention is to form, in a Group III nitride-based compound semiconductor device, a negative electrode on a surface other than a Ga-polar C-plane. In a Group III nitride-based compound semiconductor light-emitting device, there are formed, on an R-plane sapphire substrate, an n-contact layer, a layer for improving static breakdown voltage, an n-cladding layer made of a multi-layer structure having ten stacked sets of an undoped In0.1Ga0.9N layer, an undoped GaN layer, and a silicon (Si)-doped GaN layer, a multi-quantum well (MQW) light-emitting layer made of a combination of In0.25Ga0.75N well layers and GaN barrier layers stacked alternatingly, a p-cladding layer made of a multi-layer structure including a p-type Al0.3Ga0.7N layer and a p-In0.08Ga0.92N layer, and a p-contact layer (thickness: about 80 nm) made of a stacked structure including two p-GaN layers having different magnesium concentrations. Through etching, the n-contact layer having a thickness direction along the c-axis is provided with stripe-patterned microditches each having side walls, which assume a C-plane, whereby ohmic contact is established between a negative electrode and each C-plane side wall.