We present the development of Superconducting Tunnel Junction (STJ) detectors as a far-infrared single photon spectrometer, which is motivated for an application to a search for the radiative decay of the cosmic neutrino background (CνB). The photon energy spectrum from the radiative decays of CνB is expected to have a sharp edge at high energy end in a far-infrared region ranging from 14 meV to 25 meV (from 50 μm to 90 μm in wavelength) in the cosmic infrared background and the overwhelming infrared foreground from the zodiacal emission. Thus, the detector is required photon-by-photon detection with sufficiently high energy resolution, in order to gain the best signal-to-noise ratio as well as to identify the edge structure. The following two types of photon detectors are under consideration: an array of niobium/aluminum STJ (Nb/Al-STJ) pixels with a diffraction grating, and STJ using hafnium (Hf-STJ). Each Nb/Al-STJ pixel is required to be capable of detecting single photons in the far-infrared region, and the pixel array measures the photon wavelength spectrum which the diffraction grating creates. Hf-STJ is expected to achieve 2% energy resolution for single photon of 25 meV due to very small gap energy of hafnium.