  We study the phase structure of QCD at finite temperatures with two flavors
of dynamical quarks on a lattice with the size $N_s^3 \times N_t=16^3 \times
4$, using a renormalization group improved gauge action and a clover improved
Wilson quark action. The simulations are made along the lines of constant
physics determined in terms of $m_{\rm PS}/m_{\rm V}$ at zero-temperature. We
show preliminary results for the spatial string tension in the high temperature
phase.
