  The QCD phase diagram is studied in the lattice QCD simulation with the
imaginary chemical potential approach. We employ a clover-improved Wilson
fermion action of two-flavors and a renormalization-group improved gauge
action, and perform the simulation at an intermediate quark mass on a
$8^3\times 4$ lattice. The QCD phase diagram in the imaginary chemical
potential $\mu_I$ region is investigated by performing the simulation for more
than 150 points on the $(\beta,\mu_I)$ plane. We find that the Roberge-Weiss
phase transition at $\mu_I/T=\pi/3$ is first order and its endpoint is second
order, which are identified by the phase of the Polyakov loop. We determine the
pseudo-critical line from the susceptibility of the Polyakov loop modulus. We
find a clear deviation from a linear dependence of the pseudo-critical line on
$\mu_I^2$.
