  The counter jet of a short gamma-ray burst (sGRB) has not yet been observed,
while recent discoveries of gravitational waves (GWs) from a binary neutron
star (NS) merger GW170817 and the associated sGRB 170817A have demonstrated
that off-axis sGRB jets are detectable. We calculate the prompt emission from
the counter jet of an sGRB and show that it is typically 23-26 mag in the
optical-infrared band 10-10^3 sec after the GWs for an sGRB 170817A-like event,
which is brighter than the early macronova (or kilonova) emission and
detectable by LSST in the near future. We also propose a new method to
constrain the unknown jet properties, such as the Lorentz factor, opening
angle, emission radii, and jet launch time, by observing both the forward and
counter jets. To scrutinize the counter jets, space GW detectors like DECIGO
are powerful in forecasting the merger time (<~ 1 sec) and position (<~ 1
arcmin) (~ a week) before the merger.
