For the development of a better thermophotovoltaic (TPV) power generation system, it is important to develop a spectrally selective functional emitter which emits an infrared radiation spectrum to match the spectrally selective absorption spectrum of the adopted photovoltaic element. In this work, we first present an idea of a coefficient for evaluating the matching performance of such an emitter. The index compares the electric power gain for the case of the functional emitter with that for the case of a gray surface to which the same thermal energy as that to the functional surface is input. The ratio is noted as (COP)_spec. Second, we choose a high-temperature-air-oxidized nickel surface as an emitter candidate. This film system has a spectrally-selective emission function based on the radiation interference. We present a spectral experiment technique to determine the condition for fabricating the optimum film system. Third, we make the optimum film system experimentally to clarify the reality of the excellent result of (COP)_spec = 6. It is concluded that this film system is excellent in spectral performance, and that the system has a simple structure for the spectral characteristics to be controlled easily. This surface can be produced in a m²-scale large area, and it is prospective for the thermal energy engineering application.