Abstract
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}.