For the anisotropic PbSb₂Te₄ single crystal with a high hole concentration (p ≈ 3 × 10²⁰ cm^–3) and high conductivity (σ ≈ 2500 Ω^–1 cm^–1), the reflectance spectrum for a cleavage plane orthogonal to the trigonal axis C₃ is recorded in a wide spectral range from 50 to 50000 cm^–1. It is shown that, in the long-wavelength and mid-infrared regions, the reflectance can be described with consideration for the contribution of plasma vibrations and two crystal-lattice vibrations. The quantitative characteristics of these vibrations are determined. The characteristics are in satisfactory agreement with the electrical parameters. The discrepancies between the values of the hole effective mass calculated by different methods are attributed to the complex structure of the crystal’s valence band.