  We perform the first search for lepton-number-violating B+->D-l+l'+ decays,
where l and l' stand for e or mu, using 772 x 10^6 BB-bar pairs accumulated at
the Upsilon(4S) resonance with the Belle detector at the KEKB e^+e^- collider.
No evidence for these decays has been found. Assuming uniform three-body phase
space distributions for the D-l+l+ decays, we set the following upper limits on
the branching fractions at 90% confidence level: Br(B+ -> D-e+e+) < 2.6x10^-6,
Br(B+ -> D-e+mu+) < 1.8x10^-6 and Br(B+ -> D-mu+mu+) < 1.0x10^-6.
