We have measured the forward-backward charge asymmetry in the process of b-quark production in e+e- annihilation at TRISTAN. It was made possible by detecting prompt leptons from b-quarks. The obtained asymmetry is A = -0.55 +/- 0.15 +/- 0.08. If corrected for B-meson mixing effects with the assumptions given in the text, the asymmetry becomes A = -0.78 +/- 0.21 +/- 0.11, which is consistent with the prediction of the standard model, namely the assignment of the b-quark to the isospin doublet of the third quark generation.
