This paper reports on a miniaturization method of near-infrared spectrometer based on a plasmonic near-infrared photodetector. The spectrometer has an n-typed Si cantilever with an Au diffraction grating for Surface Plasmon Resonance (SPR) generation. Mechanical vibration of the cantilever provides alternation of the angle of incidence of the light to be measured. Coupled to SPR, the light energy is transduced to photocurrent. Since the different incident angle corresponds to different SPR resonant wavelength, the electrical measurement data of photocurrent during the cantilever vibration provides the whole spectrum information of the incident light. In this time, we developed a spectrometer device and measured time varying series of photocurrent during the cantilever vibration. Waveforms of the series were clearly dependent on the incident wavelength, and coherent to the SPR theory.