The mechanism of scanning tunneling microscopy (STM) observations of a surface of a silicon oxide (silica) thin film grown on crystalline silicon and its silica/silicon interface were investigated by means of semiempirical quantum chemical calculations. Cluster models representing the silica/silicon system were constructed, and their electronic structures were calculated. A previously postulated STM observation mechanism was examined in terms of the calculated energy levels of the clusters and the spatial distributions of their electron orbitals.