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Atomic resolution has been achieved by a scanning tunneling microscopy (STM) system for the first time on an oxide superconductor at cryogenic temperatures. The atomic corrugation with a periodicity of 0.35 nm and the modulation along the b-axis with a superlattice periodicity of about 2.7 nm have both been clearly observed on the cleaved a-bplane of a Bi-Sr-Ca-Cu-O single crystal. Furthermore, it is found that some features of the spectrum, including the shape of the superconducting gap, depended significantly on the distance between the probe tip and sample surface. Wide scattering of the results so far reported for various types of tunneling methods can be interpreted in terms of this distance dependence of the tunneling spectrum. We conclude that the superconducting gap structure in the spectrum is by far different from the prediction of the BCS weak-coupling theory.