  Neutron diffraction measurements under high magnetic fields have been
performed for the multiferroic compound HoMn$_{2}$O$_{5}$. At zero field,
high-temperature incommensurate magnetic (HT-ICM) -- commensurate magnetic (CM)
-- low-temperature incommensurate magnetic (LT-ICM) orders occur with
decreasing temperature, where ferroelectric polarization arises only in the CM
phase. Upon applying a magnetic field, the LT-ICM phase completely disappears
and the CM phase is induced at the lowest temperature. This field-induced CM
state is completely associated with the field-induced electric polarization in
this material [Higashiyama {\it et al}., Phys. Rev. B {\bf 72}, 064421
(2005).], strongly indicating that the commensurate spin state is essential to
the ferroelectricity in the multiferroic $R$Mn$_{2}$O$_{5}$ system.
