  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 $N$ with
GeV-scale mass. Especially, we consider the decay channel $B^+ \to \mu^+ \, N
\to \mu^+ \mu^+ \pi^-$ 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).
