A nitride-based semiconductor light-emitting device of an embodiment includes a semiconductor multilayer structure having a growing plane which is an m-plane and being made of a GaN-based semiconductor. The semiconductor multilayer structure includes a n-type semiconductor layer, a p-type semiconductor layer, a p-side electrode provided on the p-type semiconductor layer, and an active layer interposed between the n-type semiconductor layer and the p-type semiconductor layer. The ratio of the thickness of the active layer to the thickness of the n-type semiconductor layer, D, is in the range of 1.8×10−4≦D≦14.1×10−4. The area of the p-side electrode, S, is in the range of 1×102 μm2≦S≦9×104 μm2. A maximum current density which leads to 88% of a maximum of the external quantum efficiency is not less than 2 A/mm2.