The present invention relates to a method for producing an epitaxial substrate having a III-V group compound semiconductor crystal represented by the general formula In.sub.xGa.sub.yAl.sub.zN (wherein, x+y+z=1, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1) having reduced dislocation density, comprising a first step of covering with a mask made of a different material from the III-V group compound semiconductor so that only portions around points of the crystal constitute openings by using a III-V group compound semiconductor crystal having a plurality of projection shapes and a second step of growing the III-V group compound semiconductor crystal laterally by using the III-V group compound semiconductor crystal at the opening as a seed crystal. According to the present invention, an epitaxial substrate having a III-V group compound semiconductor crystal having low dislocation density and little warp is obtained.