This report describes our recent progress in the surface/interface study of high-T_c superconductors, where MBE-grown Nd_1.85Ce_0.15CuO₄(NCCO) surfaces and metal/NCCO interfaces have been investigated by photoemission spectroscopies. Experiments were performed focusing on the evolution of the surface or interface electronic structure with oxygen nonstoichiometry at the surface or interface region. The results indicate that the surface and interface electronic structures of NCCO are strongly influenced by the oxygen nonstoichiometry, which is easily caused by an inappropriate reduction process or a redox reaction at metal/NCCO interfaces due to inherently weak nature of Cu-O bonds. This problem may be universal in cuprates. The correct oxygen stoichiometry is the most crucial issue for cuprate superconductors in preparing the bulk-representative surface (interface), which is indispensable in obtaining reliable data using surface sensitive experiments and in fabricating tunnel junctions and superlattices with desirable characteristics.