An RAMO4 substrate that includes an RAMO4 monocrystalline substrate formed of a single crystal represented by general formula RAMO4, wherein R represents one or more trivalent elements selected from the group consisting of Sc, In, Y, and lanthanoid elements, A represents one or more trivalent elements selected from the group consisting of Fe(III), Ga, and Al, and M represents one or more divalent elements selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd. The RAMO4 monocrystalline substrate has a principal surface with a plurality of grooves. The principal surface has an off-angle θ with respect to a cleaving surface of the single crystal. The RAMO4 monocrystalline substrate satisfies tan θ≤Wy/Wx, where Wx is the width at the top surface of a raised portion between the grooves, and Wy is the height of the raised portion.