Abstract: An oxide superconductor capable of realizing a high critical current density and its manufacturing method requiring only a low temperature heat treatment. An oxide superconductor has a superconductive layer with a composition of RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, where RE stands for any one of rare earth elements including Y, Eu, Gd, Dy, Ho, Er, and Yb, which is formed on the substrate by RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and CuO phase resulting from a decomposition of RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase, in which the CuO phase and micro-defects caused by the decomposition function as pinning centers. This superconductive layer is formed by applying a solution containing organic compounds of a plurality of metallic elements for constituting the oxide superconductive layer; calcining the substrate applied with the solution to obtain a calcined body in which the organic compounds contained in the solution are thermally decomposed; heating the calcined body to produce RE.sub.1 Ba.sub.2 Cu.sub.