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A diagnostic system using short-wavelength far-infrared (FIR) lasers (40–70 μm in wavelength) is now being developed for high density and large volume plasmas. In the wavelength region, a CVD-diamond is the excellent materials for optical windows of the laser and the plasma vessel and beam splitters of a multichannel interferometer. To design these optical elements, the optical constants (refractive index n, absorption coefficient α and transmissivity T) of the CVD-diamond have been measured precisely by using FIR lasers of 48-, 57- and 71-μm in wavelength. As an example, the result for 57.1511 μm light is n = 2.383(1) ± 0.002, α = 0.19 ± 0.05 cm-1 and T = 97.5 ± 1.5% at 1.023 mm in thickness.