An epitaxial substrate for a semiconductor element, in which a group of group-III nitride layers are formed on a base substrate made of -oriented single crystal silicon such that a crystal plane of said group of group-III nitride layers is substantially in parallel with a substrate surface of said base substrate, said epitaxial substrate comprising: a first group-III nitride layer made of AIN and formed on said base substrate; a second group-III nitride layer made of Al p Ga 1-p N (0≤p<1) and formed on said first group-III nitride layer; a third group-III nitride layer expressed by a composition formula of Al q Ga 1-q N (0≤q≤1) and epitaxially formed on said second group-III nitride layer; and at least one fourth group-III nitride layer epitaxially formed on said third group-III nitride layer, wherein said first group-III nitride layer is a layer with crystal defects configured of at least one kind from a columnar or granular crystal or domain, an interface between said first group-III nitride layer and said second group-III nitride layer is shaped into a three-dimensional concave-convex surface, said third group-III nitride layer is formed as a graded composition layer in which the proportion of existence of Al in a group-III element continuously decreases progressively from a boundary portion at a boundary with said second group-III nitride layer toward said fourth group-III nitride layer, characterized in that the epitaxial substrate further comprises a layer made of AIN and provided between said third group-III nitride layer and said fourth group-III nitride layer; a density of convex portions of the first group-III nitride layer at the three-dimensional concave-convex surface is 5*10 9 /cm 2 or more and 5*10 10 /cm 2 or less, an average interval of the convex portions is 45 nm or more and 140 nm or less; and a side wall of one of the convex portions is formed by a plane or a plane of AIN. a first group-III nitride layer made of AIN and formed on said base substrate; a second group-III nitride layer made of Al p Ga 1-p N (0≤p<1) and formed on said first group-III nitride layer; a third group-III nitride layer expressed by a composition formula of Al q Ga 1-q N (0≤q≤1) and epitaxially formed on said second group-III nitride layer; and at least one fourth group-III nitride layer epitaxially formed on said third group-III nitride layer, wherein said first group-III nitride layer is a layer with crystal defects configured of at least one kind from a columnar or granular crystal or domain, an interface between said first group-III nitride layer and said second group-III nitride layer is shaped into a three-dimensional concave-convex surface, said third group-III nitride layer is formed as a graded composition layer in which the proportion of existence of Al in a group-III element continuously decreases progressively from a boundary portion at a boundary with said second group-III nitride layer toward said fourth group-III nitride layer, characterized in that the epitaxial substrate further comprises a layer made of AIN and provided between said third group-III nitride layer and said fourth group-III nitride layer; a density of convex portions of the first group-III nitride layer at the three-dimensional concave-convex surface is 5*10 9 /cm 2 or more and 5*10 10 /cm 2 or less, an average interval of the convex portions is 45 nm or more and 140 nm or less; and a side wall of one of the convex portions is formed by a plane or a plane of AIN.