  We report the result of a search for the decay $B^{-}\to\mu^{-}\bar\nu_\mu$.
The signal events are selected based on the presence of a high momentum muon
and the topology of the rest of the event showing properties of a generic
$B$-meson decay, as well as the missing energy and momentum being consistent
with the hypothesis of a neutrino from the signal decay. We find a 2.4 standard
deviation excess above background including systematic uncertainties, which
corresponds to a branching fraction of ${\cal B}(B^{-}\to\mu^{-}\bar\nu_\mu)
=(6.46 \pm 2.22 \pm 1.60 )\times10^{-7}$ or a frequentist 90% confidence level
interval on the $B^{-}\to\mu^{-}\bar\nu_\mu$ branching fraction of $[2.9,
10.7]\times 10^{-7}$. This result is obtained from a $711\ \text{fb}^{-1}$ data
sample that contains $772 \times 10^6$ $B\bar{B}$ pairs, collected near the
$\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy
$e^+ e^-$ collider.
