  We present a measurement of $R_{K^{\ast}}$, the branching fraction ratio
${{\cal B}(B\to K^\ast \mu^+ \mu^-)}$/ ${{\cal B}(B\to K^\ast e^+ e^-)}$, for
both charged and neutral $B$ mesons. The ratio for the charged case,
$R_{K{^{\ast +}}}$, is the first measurement ever performed. In addition, we
report absolute branching fractions for the individual modes in bins of the
squared dilepton invariant mass, $q^2$. The analysis is based on a data sample
of $711~\mathrm{fb}^{-1}$, containing $772\times 10^{6}$ $B\bar B$ events,
recorded at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB
asymmetric-energy $e^+e^-$ collider. The obtained results are consistent with
Standard Model expectations.
