  We report a search for the flavor-changing neutral current decay $B \to
K^{(*)} \ell^{+} \ell^{-}$ using a 29.1 fb${}^{-1}$ data sample accumulated at
the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^{+}e^{-}$
storage ring. We observe the decay process $B\to K\ell^+\ell^- (\ell = e, \mu
)$, for the first time, with a branching fraction of ${\cal B}(B \to K \ell^{+}
\ell^{-}) = (0.75^{+0.25}_{-0.21} \pm 0.09) \times 10^{-6}$.
