A spin-polarized neutron diffraction study was carried out in a multiferroic (antiferromagnetic ferroelectric) TbMnO₃. A large flipping ratio of magnetic diffraction intensity shows that helical configuration of spin ordering appears in the low-temperature ferroelectric antiferromagnetic phase, but not in the intermediate-temperature paraelectric antiferromagnetic phase. The sense of the helix is controlled by the direction of the ferroelectric polarization. The inverse Dialoshinskii-Moriya interaction, or so-called spin-current mechanism, can also explain the temperature dependence of the flipping ratio of the scattering intensity.