PURPOSE: To relatively simply obtain an oxide superconductor having high critical temperature of superconductivity, by retaining an oxide of three layer perovskite type in a specific temperature range in an oxygen ion plasma.

CONSTITUTION: Oxide of three layer perovskite type having a composition shown by the formula is retained in a temperature range of -183°C≤-≤20°C in an oxygen ion plasma to give the aimed superconductor. In the formula, A is Y, Yb, Tm, Er, Ho, Dy, Eu or Sm; 0≤x≤0.5 and 6.0≤y≤7.0. In the production method, the reason why the upper limit is 20°C is that oxygen composition in heat equilibrium is smaller than 6.9 in a temperature range exceeding 20°C, oxygen absorption will not occur than the oxygen composition even if the oxygen is being retained in an oxygen plasma for many hours and the critical temperature of superconductivity will not raise higher than 93°K. The reason why the lower temperature is limited to -183°C is that at a temperature lower than the temperature, an oxygen gas becomes a liquid, is concentrated on the surface of specimen and the specimen is not effectively exposed to the oxygen ion plasma.