A method of fabricating a nitride semiconductor light emitting device, the method comprising: forming an n-type nitride semiconductor layer; forming an active layer on the n-type nitride semiconductor layer, the forming of the active layer including forming well layers (341a, 342a, 349a) and barrier layers (341b, 349b), said barrier layers being grown under a hydrogen atmosphere at a growth temperature of 900-1040 °C; and forming a p-type nitride semiconductor layer on the active layer, wherein the active layer comprises a first quantum well structure including a first well layer, a first barrier layer on the first well layer, and a first seed layer for In supply on the first barrier layer, and a final quantum well structure including a final well layer and a final barrier layer on the final well layer, wherein the first well layer and the final well layer have a composition of In x Ga 1-x N (0≤x≤1), the first barrier layer and the final barrier layer have a composition of In y Ga 1-y N (0≤y≤1), and the first seed layer has a composition of In z Ga 1-z N (0<z<1) or a single layer of InN, wherein the final barrier layer directly contacts the p-type nitride semiconductor layer, wherein a composition ratio of In in the first seed layer is above 10% with respect to an overall composition and wherein a composition ratio of In in the final well layer is 15% or more with respect to an overall composition, and wherein the first and final barrier layers (341b, 349b) are 30 - 200 Å thick, the method further comprising: after forming the first seed layer, applying a heat treatment to the first seed layer, and after the first seed layer is formed, forming a second well layer on the first seed layer, characterised in that the heat treatment applied to the first seed layer (341 c) comprises elevating a growth temperature while the second well layer is grown. forming an n-type nitride semiconductor layer; forming an active layer on the n-type nitride semiconductor layer, the forming of the active layer including forming well layers (341a, 342a, 349a) and barrier layers (341b, 349b), said barrier layers being grown under a hydrogen atmosphere at a growth temperature of 900-1040 °C; and forming a p-type nitride semiconductor layer on the active layer, wherein the active layer comprises a first quantum well structure including a first well layer, a first barrier layer on the first well layer, and a first seed layer for In supply on the first barrier layer, and a final quantum well structure including a final well layer and a final barrier layer on the final well layer, wherein the first well layer and the final well layer have a composition of In x Ga 1-x N (0≤x≤1), the first barrier layer and the final barrier layer have a composition of In y Ga 1-y N (0≤y≤1), and the first seed layer has a composition of In z Ga 1-z N (0<z<1) or a single layer of InN, wherein the final barrier layer directly contacts the p-type nitride semiconductor layer, wherein a composition ratio of In in the first seed layer is above 10% with respect to an overall composition and wherein a composition ratio of In in the final well layer is 15% or more with respect to an overall composition, and wherein the first and final barrier layers (341b, 349b) are 30 - 200 Å thick, after forming the first seed layer, applying a heat treatment to the first seed layer, and after the first seed layer is formed, forming a second well layer on the first seed layer, characterised in that the heat treatment applied to the first seed layer (341 c) comprises elevating a growth temperature while the second well layer is grown.