A RAMO4 substrate that does not easily crack during or after the formation of group III nitride crystal includes 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 substrate has a crystal plane with a curvature radius r of 52 m or more, and a square value of correlation coefficient ρ of 0.81 or more. The curvature radius r is calculated as an absolute value from X-ray peak position ωi and measurement position Xi after the measurements of X-ray peak positions ωi at a plurality of positions Xi lying on a straight line passing through the center of the RAMO4 substrate. The correlation coefficient ρ is a measure of correlation between ω and measurement position Xi.