  We have searched for an exotic decay of orthopositronium into a single photon
and a short-lived neutral boson in the hitherto unexplored mass region above
900 ${\rm keV}/{\it c}^{2}$, by noting that this decay is one of few remaining
candidates which could explain the discrepancy of the orthopositronium
decay-rate. A high-resolution measurement of the associated photon energy
spectrum was carried out with a germanium detector to search for a sharp peak
from this two-body decay. Our negative result provides the upper-limits
of\mbox{ }$2.0 \times 10^{-4}$ on the branching ratio of such a decay in the
mass region from 847 to 1013 ${\rm keV}/{\it c}^{2}$, and excludes the
possibility of this decay mode explaining the discrepancy in the
orthopositronium decay-rate.
