In a process for preparing a thin film of oxide superconductor having a layered crystal structure by depositing each layer of said layered crystal structure on a substrate by Molecular Beam Epitaxy (MBE) method with introducing oxygen-containing gas which is exited by irradiation of microwave, improvement in that a film-forming operation by the MBE method is interrupted temporally after predetermined numbers of constituent layers which correspond to one unit crystal or less than one unit crystal are layered so that the deposited constituent layers are left in an activated oxygen atmosphere to effect a crystallization promotive operation, before next film-forming operation is restarted.