  WIMPs are promising dark matter candidates. A WIMP occasionally collides with
a mirror equipped with interferometric gravitational wave detectors such as
LIGO, Virgo, KAGRA and the Einstein Telescope (ET). When WIMPs collide with a
mirror of an interferometer, we expect that characteristic motions of the
pendulum and mirror are excited, and those signals could be extracted by highly
sophisticated sensors developed for gravitational wave detection. We analyze
the motions of the pendulum and mirror, and estimate the detectability of these
motions. For the "Thin-ET" detector, the signal-to-noise ratio may be $ 1.7
\left( \frac{ m_{\rm{DM}} }{ 100 \rm{GeV} } \right) $, where $ m_{\rm{DM}} $ is
the mass of a WIMP. We may set a more strict upper limit on the cross section
between a WIMP and a nucleon than the limits obtained by other experiments so
far when $ m_{\rm{DM}} $ is approximately lower than 0.2 GeV. We find an
order-of-magnitude improvement in the upper limit around $ m_{\rm{DM}} = 0.2
{\rm{GeV}} $.
