  We have developed the method that allows us to estimate the magnetic field
strength at the horizon of a supermassive black hole (SMBH) through the
observed polarization of optical emission of the accreting disk surrounding
SMBH. The known asymptotic formulae for the Stokes parameters of outgoing
radiation are azimuthal averaged, which corresponds to an observation of the
disk as a whole. We consider two models of the embedding 3D-magnetic field, the
regular field, and the regular field with an additional chaotic (turbulent)
component. It is shown that the second model is preferable for estimating the
magnetic field in NGC 4258. For estimations we used the standard accretion disk
model assuming that the same power-law dependence of the magnetic field follows
from the range of the optical emission down to the horizon. The observed
optical polarization from NGC 4258 allowed us to find the values 10^3 - 10^4
Gauss at the horizon, depending on the particular choice of the model
parameters. We also discuss the wavelength dependencies of the light
polarization, and possibly applying them for a more realistic choice of
accretion disk parameters.
