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We present an analysis of the exclusive$B^{+}\to\pi^{+}\pi^{-}\ell^{+}\nu_{\ell}$ decay, where $\ell$ represents anelectron or a muon, with the assumption of charge-conjugation symmetry andlepton universality. The analysis uses the full $\Upsilon(4S)$ data samplecollected by the Belle detector, corresponding to 711 fb$^{-1}$ of integratedluminosity. We select the events by fully reconstructing one $B$ meson inhadronic decay modes, subsequently determining the properties of the other $B$meson. We extract the signal yields using a binned maximum-likelihood fit tothe missing-mass squared distribution in bins of the invariant mass of the twopions or the momentum transfer squared. We measure a total branching fractionof ${{\cal B}(B^{+}\to \pi^{+}\pi^{-}\ell^{+}\nu_{\ell})= [22.7 ^{+1.9}_{-1.6}(\mathrm{stat}) \pm 3.5(\mathrm{syst}) ]\times 10^{-5}}$, where theuncertainties are statistical and systematic, respectively. This result is thefirst reported measurement of this decay.

Measurement of the Branching Fraction of the Decay $\boldsymbol{B^{+}\to\pi^{+}\pi^{-}\ell^{+}\nu_\ell}$ in Fully Reconstructed Events at Belle