  We consider the Standard Model with right-handed neutrinos to explain the
masses of active neutrinos by the seesaw mechanism. Since active neutrinos as
well as heavy neutral leptons are Majorana fermions in this case, the lepton
number violating process can be induced. We discuss the inverse neutrinoless
double beta decay $e^- e^- \to W^- W^-$ in the framework of the seesaw
mechanism and its detectability at future colliders. It is shown that the cross
section can be 17 fb for $\sqrt{s}=3$ TeV even with the stringent constraint
from the neutrinoless double beta decays if three (or more) right-handed
neutrinos exist. In such a case, the future $e^- e^-$ colliders can test lepton
number violation mediated by a right-handed neutrino lighter than about 10 TeV.
