  There are many extensions of the standard model that predict the existence of
electroweakly interacting massive particles (EWIMPs), in particular in the
context of the dark matter. In this paper, we provide a way for indirectly
studying EWIMPs through the precise study of the pair production processes of
charged leptons or that of a charged lepton and a neutrino at future 100 TeV
collider experiments. It is revealed that this search method is suitable in
particular for Higgsino, providing us the $5\sigma$ discovery reach of Higgsino
in supersymmetric model with mass up to 850 GeV. We also discuss how accurately
one can extract the mass, gauge charge, and spin of EWIMPs in our method.
