We have performed dielectric measurements and neutron diffraction experiments on the double perovskite In2NiMnO6. A ferroelectric polarization, P similar or equal to 30 mu C/m(2), is observed in a polycrystalline sample below T-N = 26 K where a magnetic phase transition occurs. The neutron diffraction experiment demonstrates that a complex noncollinear magnetic structure with " cycloidal" and " proper screw" components appears below T-N, which has the incommensurate propagation vector k = (k(a),0, k(c); k(a) similar or equal to 0.274, k(c) similar or equal to - 0.0893). The established magnetic point group 21' implies that the macroscopic ferroelectric polarization is along the monoclinic b axis. Recent theories based on the inverse Dzyaloshinskii- Moriya effect allow us to specify two distinct contributions to the polarization of In2NiMnO6. One of them is associated with the cycloidal component, p(1) proportional to r(ij) x (S-i x S-j)(perpendicular to), and the other with the proper screw component, p(2) proportional to [r(ij) center dot ( S-i x S-j)(parallel to)] A. The latter is explained by coupling between spin helicity and " ferriaxial" crystal rotation with macroscopic ferroaxial vector A, characteristic of the B- site ordered perovskite systems with out- of- plane octahedral tilting.