  We report on a measurement of the charmless semileptonic B decays,
B0->pi^-l^+nu and B0->rho^-l^+nu, based on 140 fb^{-1} data collected with the
Belle detector at the KEKB e^+e^- asymmetric collider. In this analysis,
accompanying B mesons are reconstructed from semileptonic B->D^(*)lnu decays,
which enables us to detect the signal with high purity and with marginal
statistics. We found branching fractions of Br(B0->pi^-l^+nu)=(1.76 \pm 0.28
\pm 0.20 \pm 0.03)10^{-4} and Br(B0->rho^-l^+nu)=(2.54 \pm 0.78 \pm 0.85 \pm
0.30)10^{-4}, where the errors are statistical, experimental systematic,
systematic due to form-factor uncertainties. We present also the branching
fractions in three q^2 intervals; q^2 < 8, 8 \leq q^2 < 16, q^2 \geq 16
GeV^2/c^2, for each decay mode. Based on these results, the magnitude of the
Cabibbo-Kobayashi-Maskawa matrix element V_{ub} is extracted. All of the
presented results are preliminary.
