Spectral dependencies of light reflection and absorption coefficients on photon energy for layered tetradymit-like n-PbBi4Te7 semiconductor doped with silver (or cadmium) and intrinsic defects have been investigated at T = 300 K. The compound under study was established to exhibit a narrow direct forbidden gap. It was found that its optical band gap Eg opt = 0.31 eV, an energy gap parameter Eg0 between absolute extremums of the valence and conduction bands in the sample with the Hall electron concentration of 5.45·1020 cm–3 was equal to 0.08 – 0.12 eV, and a mdn/mdp ratio (effective masses of electron and hole states densities) was equal to about unity. No other subbands located near the absolute extremum of conduction band was revealed in PbBi4Te7. The electron dispersion law was also found to be non-parabolic in this matter. The law involved can be described in the context of Kane non-parabolisity model taking into account an exchange interaction of free electrons. It was shown that the most probable value of the Eg0 parameter fell within the range from 0.16 to 0.24 eV for PbBi4Te7 with nondegenerate free electron gas.