The electron spectrum and optical absorption of antiferromagnetic chromium are calculated within the relativistic Green function method, being an analogue of the KKR method. The influence of the spin-orbit interaction and the spin polarization on the electron spectrum structure is analysed. An additional optical absorption maximum in the infrared region that is absent in the non-relativistic calculation is shown to appear on the theoretical curve when the relativistic effects are taken into account. The energy positions of this and other maxima due to antiferromagnetic ordering at h omega <0.8 eV agree well with the experimental data.