The weak itinerant ferromagnet MnSi orders with a left-handed helical spin structure below T_C = 29 K. The helicity with a vector m = [S ₁× S₂]/S₂ along the crystallographic axis is realized by an antisymmetric Dzyaloshinski-Moriya interaction. The small angle diffraction study with polarized neutrons on a single MnSi crystal was performed within the temperature range from 10 K to TC and the magnetic field B from 1 to 350 mT. The single crystal was oriented in such a way that two axes and were set in a plane perpendicular to the incident beam. Four major diffraction peaks at ±q₁ and ±q₂ along the axes and four minor peaks at q = ±q₁±q₂ were observed. The intensity I_p = I(+/P_o) + I(-P _o), the polarization P_p = [I(+P_o) -I(-P _o)]/Ip and the position q_p of the peaks were measured as a function of the temperature and the magnetic field. From intensity of the peaks the chiral critical exponent is obtained as β= 0.47 ±0.04.