  We investigate the finite-temperature phase structure and the scaling of the
chiral condensate in lattice QCD with two degenerate light quarks, using a
renormalization group improved gauge action and the Wilson quark action. We
obtain a phase diagram which is consistent with that from the parity-flavor
breaking scenario. The scaling relation for the chiral condensate assuming the
critical exponents and the scaling function of the three dimensional O(4) model
is remarkably satisfied for a wide range of parameters. This indicates that the
chiral transition in two flavor QCD is of second order in the continuum limit.
