  Muon spin relaxation ($\mu$SR) and nuclear magnetic resonance (NMR)
experiments revealed that the spin singlet state with an excitation gap of
$\sim$200 K is realized from $S = 1/2$ Nb$_4$ tetrahedral clusters in a cluster
Mott insulator GaNb$_4$S$_8$. The intercluster cooperative phenomenon to the
singlet state at $T_\mathrm{S} = 32$ K is triggered by intracluster Jahn-Teller
type structural instability developed from $\sim$$3T_\mathrm{S}$. Referring to
the lattice symmetry, the formation of Nb$_8$ octamer (Nb$_4$--Nb$_4$ bond) is
suggested.
