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 8^3×4 lattice. The QCD phase diagram in the imaginary chemical potential μ_I region is investigated by performing the simulation for more than 150 points on the (β,μ_I) plane. We find that the Roberge-Weiss phase transition at μ_I/T=π/3 is first order and its endpoint is second order, which are identified by the phase of the Polyakov loop. We determine the pseudocritical line from the susceptibility of the Polyakov loop modulus. We find a clear deviation from a linear dependence of the pseudocritical line on μ^2_1.