  Prospects are examined for the detection of a slow decay of the lightest
neutralino (or any other longlived particles) at the CERN LHC and at Very Large
Hadron Collider (VLHC). We first point out that such hadron colliders will
become the ``neutralino factory'' producing 10^6--10^9 neutralinos/yr, if
gluinos and/or squarks actually exist below O(1) TeV. The lightest neutralino
(\neu), usually assumed to be stable, will be unstable if lighter
superparticles such as the gravitino (\gra) or axino (\axi) exist, or R-parity
is not conserved. The decay signal would, however, be missed in usual collider
experiments, particularly when the decay mostly occurs outside the detector. In
order to search for such a slow decay of \neu, we propose a dedicated
experiment where the collision products are dumped by a thick shield, which is
followed by a long decay tunnel. The decay product of \neu can be detected by a
detector located at the end of the tunnel. The slow arrival time and the large
off angle (to the direction of the interaction point) of the decay product will
provide a clear signature of slowly decaying \neu's. One can explore the decay
length (c\tau) in a wide range, i.e., 0.2 m to 1x10^5 km for \mneu=25 GeV and 1
m to 2 km for \mneu=200 GeV at the LHC. This corresponds to the range of the
SUSY breaking scale \rtF=2x10^5 to 2x10^7 GeV in case of the \neu\to\gamma\gra
decay predicted in gauge-mediated SUSY breaking models. At VLHC, one can extend
the explorable range of \mneu up to 1000 GeV, and that of \rtF up to 1x10^8
GeV. In case of the \neu\to\gamma\axi decay, the Peccei- Quinn symmetry
breaking scale F_a can be explored up to 5x10^11 GeV. The mass of the decaying
particle can be determined by using the correlation between the energy and the
arrival time of the decay product.
