Аннотация:Our spectroscopic observations with FORS1 at 8.2-m VLT telescope (Paranal, Chile) lead to detection of magnetic field in the X-ray binary Cyg X-1. That is the first successful attempt of measuring magnetic field in a binary with a black hole. The value of the mean longitudinal magnetic field in optical component (O9.7 Iab supergiant) changes regularly with the orbital phase reaching its maximum of 130 G (σ\ap20 G). The measurements based on Zeeman effect were carried through over all observed supergiant photosphere absorption spectral lines. Similar measurements over the emission line He II λ 4686 Å yielded a value of several hundreds Gauss of a smaller significance level. The system Doppler tomogram we build over the line profiles shows that He II λ 4686 Å originates in the outer regions of the accretion structure. The values measured correspond, in the frame of the disc accretion standard model, to a near-black-hole field of \tilde 10\^8-10\^9 G and may be responsible for the observed Cyg X-1 X-ray flickering. Also some consequences of such magnetic field existence in Cyg X-1 optical component photosphere were suggested.