Spectral control of thermal radiation emitted from rectangular micro-cavities made on a metal surface was investigated through numerical simulation and experiment. In the numerical simulation, thermal radiation from solid surface was dealt as hemispherical emission from point sources, and the Maxwell's equations were solved using the cubic interpolated propagation method. It was demonstrated that spherical waves emitted from inside of a cavity were spectrally selected, and that the emittance could be increased around the wavelength corresponding to the standard mode of cavity resonance. Furthermore, in experiment using rectangular micro-cavities (0.5x0.5x0.5 μm3) made periodically on Ni specularly-polished surface, spectral emittance was measured in the near-infrared region. The experimental results disclosed that the emissive power only in the range of shorter wavelength than 1.2 μm was increased by the micro-cavities that played a role of a wave guide to produce cutoff effect clearly.