  Spin fluctuations in the triangular-lattice Heisenberg antiferromagnet
NaCrO$_2$ are investigated by means of quasi-elastic neutron scattering with
high energy resolution and wide energy band. Two components with the following
features are captured separately. They are pronounced at intermediate
temperatures of 20-50 K. One with quite an extended lifetime corresponding to
$\sim$$0.001E_{\rm ex}$ ($E_{\rm ex}$ the exchange energy) nearly disappears at
low temperature 10 K, and the other with an extended lifetime $\sim$$0.01E_{\rm
ex}$ survives there, identified as free $Z_2$ vortex and $Z_2$-vortex pair,
respectively, in harmony with the $Z_2$ vortex theory.
