A cleaning process of a (100) MgO surface is investigated on the basis of thermal desorption spectroscopy. All the desorbed species are oxygen-containing molecules. A contamination-free MgO surface is obtained with an in situ annealing process at fairly low desorption temperature (640 o C) for 2 h. Pre-annealing in O 2 atmosphere of 600 mTorr, which is performed to compensate for the surface defects produced by desorbing the oxygen-containing molecules, enables us to provide high-quality BaKBiO superconducting films.