  We study Faraday rotation in the jet of M87 inside the Bondi radius using
eight Very Long Baseline Array data sets, one at 8 GHz, four at 5 GHz, and
three at 2 GHz. We obtain Faraday rotation measures (RMs) measured across the
bandwidth of each data set. We find that the magnitude of RM systematically
decreases with increasing distance from the black hole from 5,000 to 200,000
Schwarzschild radii. The data, showing predominantly negative RM sign without
significant difference of the RMs on the northern and southern jet edges,
suggest that the spatial extent of the Faraday screen is much larger than the
jet. We apply models of hot accretion flows, thought to be prevalent in active
galactic nuclei having relatively low luminosity such as M87, and find that the
decrease of RM is described well by a gas density profile $\rho \propto
r^{-1}$. This behavior matches the theoretically expected signature of
substantial winds, nonrelativistic un-collimated gas outflows from hot
accretion flows, which is consistent with the results of various numerical
simulations. The pressure profile inferred from the density profile is flat
enough to collimate the jet, which can result in gradual acceleration of the
jet in a magneto-hydrodynamical process. This picture is in good agreement with
the observed gradual collimation and acceleration of the M87 jet inside the
Bondi radius. The dominance of negative RMs suggests that jet and wind axis are
misaligned such that the jet emission exposes only one side of the toroidal
magnetic fields permeating the winds.
