We present a measurement of the B^0-B^0bar mixing parameter Delta m_d using neutral B meson pairs in a 29.1 fb^{-1} data sample collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We exclusively reconstruct one neutral B meson in the semileptonic B^0 \to D^{*-}\ell^+\nu decay mode and identify the flavor of the accompanying B meson from its decay products. From the distribution of the time intervals between the two flavor-tagged B meson decay points, we obtain Delta m_d = (0.494 +- 0.012 +- 0.015) ps^{-1}, where the first error is statistical and the second error is systematic.