Disclosed is a crack-free epitaxial substrate which uses a silicon substrate as a base substrate and has excellent withstand voltage. Specifically disclosed is an epitaxial substrate which is obtained by forming a group of group III nitride layers on a (111) single crystal silicon base substrate such that the (0001) crystal plane is generally parallel to the substrate surface. The epitaxial substrate comprises a buffer layer that comprises a plurality of first lamination units, each of which includes a composition modulation layer, wherein first composition layers that are formed from AlN and second composition layers that are formed from AlxGa1-xN are alternately laminated, and a first intermediate layer that is formed from AlyGa1-yN (wherein 0< = y < 1). When the respective numbers of the first composition layers and the second composition layers laminated are represented by n and the value of x in the i-th second composition layer from the base substrate is represented by x(i), x(1) >= x(2)> = ... >= x(n-1)> = x(n) and x(1) > x(n) are satisfied. The second composition layers are coherent to the first composition layers, and the first intermediate layer is coherent to the composition modulation layer.