an electronic device substrate includes a spinel epitaxial film formed on a silicon single - crystal substrate and an oxide superconductor layer formed on the spinel film . the oxide superconductor layer is represented by formula p x y cu z o . sub . δ and contains at least one element of bi and tl as p and at least one element of sr and ba as q . composition ratios fall within ranges of 0 . 08 ≦ x /≦ 0 . 41 , 0 . 29 ≦ y /≦ 0 . 47 and 1 ≦ q / ca ≦ 3 .