Semigrand canonical (SGC) Monte Carlo (MC) simulation has been performed for In_xGa_I-XN thin film pseudomorphic to the GaN (0001) substrate. In the SGC ensemble the chemical potential differences, Δμ=μ_In- μ_Ga, is given. The species identity changes are attempted in the SGC MC simulation with the total number of particles being fixed. Free energies of Ga-rich and In-rich phases at a temperature T=800K have been calculated by integrating the composition (the mole fraction, x_In=n_In/(n_In+n_Ga)) by Δμ in this case. Once one obtained a phase equilibrium condition Δμ(T) at T, one can search the phase equilibrium condition at the adjacent temperature T+dT with the help of d(Δμ)/dT= -Δh/TΔx, where Δh and Δx are enthalpy difference and the composition difference, respectively. We have applied this method, the Gibbs-Duhem integration technique, to outline the boundary of the In_xGa_1-xN/GaN alloy phase diagram. We have looked into the detailed structure of the In_xGa_1-xN/GaN along the Ga-rich boundary and found the superlattice structure.