  Constraints on the heavy sterile neutrino mixing angles are studied in the
framework of a minimal supersymmetric ${\rm SO}(10)$ model with {\it double
see-saw mechanism}. A new singlet matter in addition to the right-handed
neutrinos is introduced to realize the double see-saw mechanism. The minimal
${\rm SO}(10)$ model gives an unambiguous Dirac neutrino mass matrix, which
enables us to predict the masses and the mixing angles in the enlarged $9
\times 9$ neutrino mass matrix. Mixing angles between the light Majorana
neutrinos and the heavy sterile neutrinos are shown to be within the LEP
experimental bound on all ranges of the Majorana phases.
