  We study the finite-temperature phase structure and the transition
temperature of QCD with two flavors of dynamical quarks on a lattice with the
temporal size $N_t=4$, using a renormalization group improved gauge action and
the Wilson quark action improved by the clover term. The region of a
parity-broken phase is identified, and the finite-temperature transition line
is located on a two-dimensional parameter space of the coupling ($\beta=6/g^2$)
and hopping parameter $K$. Near the chiral transition point, defined as the
crossing point of the critical line of the vanishing pion mass and the line of
finite-temperature transition, the system exhibits behavior well described by
the scaling exponents of the three-dimensional O(4) spin model. This indicates
a second-order chiral transition in the continuum limit. The transition
temperature in the chiral limit is estimated to be $T_c = 171(4)$ MeV.
