The polarization analysis for the (200)M1 reflection has been made on a twin free single crystal of NiS 2 at ∼5 K. The final polarization ( P f ) was measured as a function of the rotating angle (ϕ) about the scattering vector. ( P f is approximately expressed by -0.95 cos 2ϕ.) The antiferromagnetic structure with respect to the first kind ordering was determined under the assumption that a particular domain distribution is not realized in the sample. The previously proposed non-collinear structure was confirmed. A weakferromagnetic moment more than 0.01 (µ B ) was observed on each nickel ion forming the pyrite structure by measuring the flipping ratio of the (400) nuclear reflection. Mechanisms for weakferromagnetism are discussed.