  We present a toy model study of the high temperature deconfining transition
in Yang-Mills theory as a breakdown of the confinement condition proposed by
Kugo and Ojima. Our toy model is a kind of topological field theory obtained
from the Yang-Mills theory by taking the limit of vanishing gauge coupling
constant $g_{\rm YM}\to 0$, and therefore the gauge field $A_\mu$ is
constrained to the pure-gauge configuration $A_\mu=g^{\dagger}\partial_\mu g$.
At zero temperature this model has been known to satisfy the confinement
condition of Kugo and Ojima which requires the absence of the massless
Nambu-Goldstone-like mode coupled to the BRST-exact color current. In the
finite temperature case based on the real-time formalism, our model in 3+1
dimensions is reduced, by the Parisi-Sourlas mechanism, to the ``sum'' of
chiral models in 1+1 dimensions with various boundary conditions of the group
element $g(t,x)$ at the ends of the time contour. We analyze the effective
potential of the $SU(2)$ model and find that the deconfining transition in fact
occurs due to the contribution of the sectors with non-periodic boundary
conditions.
