Analysis of the polarization properties in the conical jet of M87 revealing a helical magnetic field configuration. The central frequency is 11 GHz. At the top, there is the high angular resolution (0.″09) Stokes I image showing the double-helix structure between knots D and I. The two lines were obtained by fitting slices perpendicular to the jet axis to two Gaussians. The middle image shows the LIC streamline image of the component of the magnetic field parallel to the plane of the sky over the fractional polarization map. The magnetic field lines approximately follow the double-helix structure, which suggests we are observing the poloidal component of the magnetic field in the jet. Note the increase of the fractional polarization when emission from the edges of the jet width can be well resolved. The bottom image shows a close-up of the region enclosing knots E and F, the region where the filaments appear more separated. Again, we show the streamline image of the magnetic field parallel to the plane of the sky, but now, the color scale shows the values of the Faraday depth obtained from our modeling of the Stokes parameters Q(λ2) and U(λ2). The sign of the Faraday depth traces the direction of the magnetic field along the line of sight. Thus, it can be clearly seen that in the region where we are able to separate emission from both edges of the jet, the magnetic field has opposite directions, strongly suggesting a toroidal component. All of these characteristics are strongly supporting the presence of a helical configuration in the M87 jet.