A method of manufacturing an epitaxial substrate for semiconductor device, said method comprising: a channel layer formation step of epitaxially forming a channel layer on a base substrate, said channel layer being made of a first group-III nitride having a composition of In x1 Al y1 Ga z1 N, where x1+y1+z1=1, that contains at least Ga; a barrier layer formation step of epitaxially forming a barrier layer on said channel layer, said barrier layer being made of a second group-III nitride having a composition of In x2 Al y2 Ga z2 N, where x2+y2+z2=1, that contains at least In and Al; wherein in said channel layer formation step, the composition of said first group-III nitride is selected from a range determined by x1=0 and 0≤y1≤0.3, in said barrier layer formation step, the composition of said second group-III nitride is selected, in a ternary phase diagram for InN, AlN, and GaN, from a range that is enclosed by straight lines represented by the following expressions for x2 and z2 and that is determined in accordance with the composition of said first group-III nitride, and said barrier layer is formed with a thickness of 3 nm or less, wherein the expressions for x2 and z2 are: x 2 = y 2 − 0.27 + 0.5 y 1 1.78 = − z 2 − 0.73 − 0.5 y 1 2.78 x 2 = y 2 − 0.4 + 0.6 y 1 1.78 = − z 2 − 0.6 − 0.6 y 1 2.78 z 2 = 0.4 z 2 = 0, characterised in that the method comprises a main insulating layer formation step of forming a main insulating layer on said barrier layer, said main insulating layer being made of silicon nitride, wherein in said main insulating layer formation step, said main insulating layer is formed with a thickness of 3 nm or less by MOCVD at a formation temperature of 750°C or more and 850°C or less, the method further comprising: a sub insulating layer formation step of epitaxially forming a sub insulating layer on said barrier layer, said sub insulating layer being made of a group-III nitride with insulating properties and having a thickness of 2.5 nm or less, wherein said main insulating layer is formed on said sub insulating layer, and wherein said sub insulating layer is made of AlN. a channel layer formation step of epitaxially forming a channel layer on a base substrate, said channel layer being made of a first group-III nitride having a composition of In x1 Al y1 Ga z1 N, where x1+y1+z1=1, that contains at least Ga; a barrier layer formation step of epitaxially forming a barrier layer on said channel layer, said barrier layer being made of a second group-III nitride having a composition of In x2 Al y2 Ga z2 N, where x2+y2+z2=1, that contains at least In and Al; wherein in said channel layer formation step, the composition of said first group-III nitride is selected from a range determined by x1=0 and 0≤y1≤0.3, in said barrier layer formation step, the composition of said second group-III nitride is selected, in a ternary phase diagram for InN, AlN, and GaN, from a range that is enclosed by straight lines represented by the following expressions for x2 and z2 and that is determined in accordance with the composition of said first group-III nitride, and said barrier layer is formed with a thickness of 3 nm or less, wherein the expressions for x2 and z2 are: x 2 = y 2 − 0.27 + 0.5 y 1 1.78 = − z 2 − 0.73 − 0.5 y 1 2.78 x 2 = y 2 − 0.4 + 0.6 y 1 1.78 = − z 2 − 0.6 − 0.6 y 1 2.78 z 2 = 0.4 z 2 = 0, characterised in that the method comprises a main insulating layer formation step of forming a main insulating layer on said barrier layer, said main insulating layer being made of silicon nitride, wherein in said main insulating layer formation step, said main insulating layer is formed with a thickness of 3 nm or less by MOCVD at a formation temperature of 750°C or more and 850°C or less, the method further comprising: a sub insulating layer formation step of epitaxially forming a sub insulating layer on said barrier layer, said sub insulating layer being made of a group-III nitride with insulating properties and having a thickness of 2.5 nm or less, wherein said main insulating layer is formed on said sub insulating layer, and wherein said sub insulating layer is made of AlN. the composition of said second group-III nitride is selected, in a ternary phase diagram for InN, AlN, and GaN, from a range that is enclosed by straight lines represented by the following expressions for x2 and z2 and that is determined in accordance with the composition of said first group-III nitride, and said barrier layer is formed with a thickness of 3 nm or less,