  We report a measurement of the ratios of branching fractions
$\mathcal{R}(D^{(*)}) = {\cal B}(\bar{B} \to D^{(*)} \tau^-
\bar{\nu}_{\tau})/{\cal B}(\bar{B} \to D^{(*)} \ell^- \bar{\nu}_{\ell})$ where
$\ell$ denotes an electron or a muon. The results are based on a data sample
containing $772 \times 10^6$ $B\bar{B}$ events recorded at the $\Upsilon(4S)$
resonance with the Belle detector at the KEKB $e^+ e^-$ collider. The tag-side
B meson is reconstructed in a semileptonic decay mode, and the signal-side
${\tau}$ is reconstructed in a purely leptonic decay. The results are
$\mathcal{R}(D) = 0.307 \pm 0.037 \pm 0.016$ and $\mathcal{R}(D^*)=0.283 \pm
0.018 \pm 0.014$, where the first uncertainties are statistical and the second
are systematic. These results are in agreement with the Standard Model
predictions within 0.2 and 1.1 standard deviations, respectively.
