  The quark-gluon mixed condensate g<\bar{q} \sigma G q> is studied using
SU(3)c lattice QCD with the Kogut-Susskind fermion at the quenched level. Using
the lattices as \beta = 6.0 with 16^3 * N_t (N_t=16,12,10,8,6,4), \beta = 6.1
with 20^3 * N_t (N_t=20,12,10,8,6) and \beta = 6.2 with 24^3 * N_t
(N_t=24,16,12,10,8) in high statistics of 100-1000 gauge configurations, we
perform accurate measurement of the thermal effects on g<\bar{q} \sigma G q> as
well as <\bar{q}q> in the chiral limit. We find that the thermal effects on
both the condensates are very weak except for the vicinity of T_c, while both
the condensates suddenly vanish around T_c \simeq 280 MeV, which indicates
strong chiral restoration near T_c. We also find that the ratio m_0^2 =
g<\bar{q} \sigma G q>/ <\bar{q}q> is almost independent of the temperature even
in the very vicinity of T_c, which means that these two different condensates
obey the same critical behavior. This nontrivial similarity between them would
impose constraints on the chiral structure of the QCD vacuum near T_c.
