  The electroweak penguin B decays mediated by $b \to s \ell^+ \ell^- $
transitions are flavour-changing neutral current processes, and are thus
sensitive to new physics owing to potential contributions of heavy particles in
the quantum loop. Recently, LHCb has obtained interesting results, where the
possible hints of lepton family universality violation (LFUV) could be seen. We
report a new measurement of the LFUV observable $R_{K}$, the ratio of branching
fractions of $B\to K\mu^{+}\mu^{-}$ to $B\to Ke^{+}e^{-}$, based on the full
data sample recorded by Belle at the $\Upsilon(4S)$ resonance from $e^+e^-$
collisions produced by the KEKB collider. We also report results on lepton
flavor violating $B$ decays, $B^{+}\to K^{+}\mu^{\pm}e^{\mp}$ and $B^{0}\to
K_{S}^{0}\mu^{\pm}e^{\mp}$. The $B$-factory at KEK is also a $\tau$ factory,
creating a copious amount of $\tau\tau$ pairs. We have used these data to look
for lepton and baryon number violating $ \tau$ decays
$\tau^{-}\to\overline{p}e^{+}e^{-}$, $pe^{-}e^{-}$, $\overline{p}e^{+}\mu^{-}$,
$\overline{p}e^{-}\mu^{+}$, $\overline{p}\mu^{+}\mu^{-}$, and
$p\mu^{-}\mu^{-}$. Results of the search are also reported.
