  Small angle neutron inelastic scattering measurement has been performed to
study the magnon dispersion relation in the field-induced-ferromagnetic phase
of the noncentrosymmetric binary compound MnSi. For the magnons propagating
parallel or anti-parallel to the external magnetic field, we experimentally
confirmed that the dispersion relation is asymmetrically shifted along the
magnetic field direction. This magnon dispersion shift is attributed to the
relativistic Dzyaloshinskii-Moriya interaction, which is finite in
noncentrosymmetric magnets, such as MnSi. The shift direction is found to be
switchable by reversing the external magnetic field direction.
