A bipolar (southward-then-northward) signature of the Bz component at the leading edge of earthward jets in the magnetotail has often been reported. Here we try to reproduce this magnetic field feature by a three-dimensional reconnection (finite extent in the dawn-dusk direction) in the presence of a nonzero guide field (dawn-dusk magnetic component). Setting the guide field intensity by referring to earlier reports (Slavin et al., 2003) and setting the dawn-dusk extent of the reconnection region to an often quoted value (Angelopoulos et al., 1997), we managed to reproduce the bipolar signatures in three-dimensional (Hall) MHD simulation runs. Redirecting the argument, with the typical guide field intensity of By = 4 nT in the magnetotail, the southward peak value of Bz = -3 nT in the bipolar signature may be taken as the evidence that the dawn-dusk width of the reconnection region is ~4 RE (RE: Earth radii).