A method of producing a Mg-doped high-temperature superconductor having low superconducting anisotropy, comprising depositing or applying a superconductor comprising Cu1−xMx(Ba1−ySry)2(Ca1−zMgz)n−1CunO2n+4−w (in which M is at least one selected from a group consisting of Tl, Hg, Bi, Pb, Au, In, Sn, Mg, Ag, Mo, Re, Os, Cr, V, Fe, and lanthanide elements, 0≦x<1, 0≦y<1, 0≦z<1, 0≦w≦4, and 3≦n≦16), or Cu1−xMx(Ba1−y−mSryRm)2(Ca1−zMgz)n−1CunO2n+4−w (in which R is at least one selected from a group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, with 0≦m<1, 0≦y+m≦1, 0≦x<1, 0≦y<1, 0<z<1, 0≦w≦4, and 3≦n≦16), or starting materials of the superconductor, on a single-crystal substrate or crystal oriented film substrate, and sealing the substrate in an oxidation resistant capsule to obtain a single-crystal or crystal oriented film aligned along at least an a-axis and c-axis with a high critical current density Jc.