Heavy Majorana neutrinos are predicted in addition to ordinary active neutrinos in the models with the seesaw mechanism. We investigate the lepton number violation (LNV) in B decays induced by such a heavy neutrino Nwith GeV- scale mass. Especially, we consider the decay channel B+ -> mu(+) N -> mu(+) mu(+) pi(-) and derive the sensitivity limits on the mixing angle Theta(mu) by the future search experiments at Belle II and in e(+) e(-) collisions at the Future Circular Collider (FCC-ee). (C) 2016 The Author(s). Published by Elsevier B.V.