  We report a measurement of ratio ${\cal R}(D^*) = {\cal B}(\bar{B}^0
\rightarrow D^{*+} \tau^- \bar{\nu}_{\tau})/{\cal B}(\bar{B}^0 \rightarrow
D^{*+} \ell^- \bar{\nu}_{\ell})$, where $\ell$ denotes an electron or a muon.
The results are based on a data sample containing $772\times10^6$ $B\bar{B}$
pairs recorded at the $\Upsilon(4S)$ resonance with the Belle detector at the
KEKB $e^+ e^-$ collider. We select a sample of $B^0 \bar{B}^0$ pairs by
reconstructing both $B$ mesons in semileptonic decays to $D^{*\mp} \ell^{\pm}$.
We measure ${\cal R}(D^*)= 0.302 \pm 0.030({\rm stat)} \pm 0.011({\rm syst)}$,
which is within $1.6 \sigma$ of the Standard Model theoretical expectation,
where $\sigma$ is the standard deviation including systematic uncertainties.
