  We demonstrate that the system of localized magnetic moments in
HoNi$_2$B$_2$C can be described by the 4-positional clock model. This model, at
a proper choice of the coupling constants, yields several metamagnetic phases
in magnetic field at zero temperature in full agreement with the experimental
phase diagram. The model incorporates couplings between not nearest neighbors
in the direction perpendicular to the ferromagnetic planes. The same model
leads to a c-modulated magnetic phase near the Curie temperature. The
theoretical value of the modulation wave-vector agrees surprisingly well with
that observed by the neutron diffraction experiment without new adjustable
parameters.
