  We report 7Li NMR studies of single crystals of triangular-lattice Heisenberg
antiferromagnet Li7RuO6. Slow critical divergence with a wide critical region
of |T/TN - 1|< 7 was observed in 7Li nuclear spin-lattice relaxation rate. The
slowing down of staggered spin fluctuations was analyzed in a renormalized
classical region of a two-dimensional triangular-lattice non-linear sigma
model. A spin stiffness constant was found to reduce to about 20 % from the
value in a spin-wave approximation. The effect of spin frustration, e.g., Z2
vortex excitations on the critical phenomena is suggested.
