An epitaxial substrate 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 buffer layer formed of a first lamination unit and a second lamination unit being alternately laminated such that each of an uppermost portion and a lowermost portion of said buffer layer is formed of said first lamination unit; and a crystal layer formed on said buffer layer, wherein said first lamination unit includes: a composition modulation layer formed of a first unit layer and a second unit layer having different compositions being alternately and repeatedly laminated so that a compressive strain exists therein, wherein a first group-III nitride of said first unit layer has an in-plane lattice constant, under a strain-free state, smaller than that of a second group-III nitride of said second unit layer, each of said second unit layers is formed so as to be in a coherent state relative to said first unit layer; and a first intermediate layer for enhancing said compressive strain existing in said composition modulation layer, wherein said first intermediate layer is made of a third group-III nitride and formed so as to be in a coherent state relative to said composition modulation layer; said second lamination unit is a second intermediate layer that is substantially strain-free, characterized in that said second intermediate layer is made of a fourth group-III nitride whose in-plane lattice constant under a strain-free state is smaller than that of said third group-III nitride. a buffer layer formed of a first lamination unit and a second lamination unit being alternately laminated such that each of an uppermost portion and a lowermost portion of said buffer layer is formed of said first lamination unit; and a crystal layer formed on said buffer layer, wherein said first lamination unit includes: a composition modulation layer formed of a first unit layer and a second unit layer having different compositions being alternately and repeatedly laminated so that a compressive strain exists therein, wherein a first group-III nitride of said first unit layer has an in-plane lattice constant, under a strain-free state, smaller than that of a second group-III nitride of said second unit layer, each of said second unit layers is formed so as to be in a coherent state relative to said first unit layer; and a first intermediate layer for enhancing said compressive strain existing in said composition modulation layer, wherein said first intermediate layer is made of a third group-III nitride and formed so as to be in a coherent state relative to said composition modulation layer; a composition modulation layer formed of a first unit layer and a second unit layer having different compositions being alternately and repeatedly laminated so that a compressive strain exists therein, wherein a first group-III nitride of said first unit layer has an in-plane lattice constant, under a strain-free state, smaller than that of a second group-III nitride of said second unit layer, each of said second unit layers is formed so as to be in a coherent state relative to said first unit layer; and a first intermediate layer for enhancing said compressive strain existing in said composition modulation layer, wherein said first intermediate layer is made of a third group-III nitride and formed so as to be in a coherent state relative to said composition modulation layer; said second lamination unit is a second intermediate layer that is substantially strain-free, characterized in that said second intermediate layer is made of a fourth group-III nitride whose in-plane lattice constant under a strain-free state is smaller than that of said third group-III nitride.