An oxide superconductor includes: REBa2Cu3O7-x (RE being one element selected from a “RE element group” of Pr, Nd, Sm, Eu, Gd, Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu). The RE includes at least three, types of metallic elements (M1, M2, and M3), and the three types of metallic elements are any element of the RE element group selected in order. In an oxide system satisfying R(1)≤20 mol % and R(M2)≥60 mol % and R(M3)≤20 mol %, R(M1) being an average metallic element ratio of M1 in M1+M2+M3, SD(Ms)>0.15 is satisfied at a position at 50% of an average film thickness of a cross section including the c-axis, Ms being the metallic element of not larger of R(M1) and R(M3), SD(Ms) being a standard deviation/average value of a concentration of Ms.