A magnesium oxide single crystal having controlled crystallinity has a subboundary, and ranges of variation of diffraction line positions, as measured for reciprocal lattice maps with respect to a region surrounded by the same subboundary, with the range of the variation of 1.times.10.sup.-3 to 2.times.10.sup.-2 degree of on .DELTA..omega. coordinates, and with the range of the variation of 4.times.10.sup.-4 to 5.times.10.sup.-3 degree on 2.theta. coordinates.