A GaN-based compound semiconductor device formed by sequentially forming, on a substrate, a GaN-based buffer layer and a GaN-based compound semiconductor layer. Al_xGa_1-xN_1-yP_y or Al_xGa_1-xN_1-yAs_y (0x1, 0y1) is used as the GaN-based buffer layer. N in Al_xGa_1-xN is partially substituted by P or As, whereby a buffer layer is grown at a high temperature. Thus, a difference in processing temperature between the process for growing a buffer layer and processes before and after the process is reduced. The GaN-based compound semiconductor layer formed on the buffer layer comprises a GaN-based layer, an n-type clad layer, a light-emitting layer, and a p-type clad layer. A multiple quantum well (MQW) layer formed from GaNP or GaNAs and GaN is inserted between the GaN-based layers, thereby reducing a dislocation density of the GaN-based layers.