  We investigate the temperature dependence of the instanton contents of gluon
fields, using unquenched lattice QCD and the cooling method. The instanton size
parameter deduced from the correlation function decreases from 0.44fm below the
phase-transition temperature $T_c$ ($\approx 150$MeV) to 0.33fm at 1.3 $T_c$.
The instanton charge distribution is Poissonian above $T_c$, but it deviates
from the convoluted Poisson at low temperature. The topological susceptibility
decreases rapidly below $T_c$, showing the apparent restoration of the $U(1)_A$
symmetry already at $T \approx T_c$.
