In order to elucidate phenomena of Venus atmosphere, an atmospheric general circulation model (AGCM) for Venus is being developed on the basis of AFES (AGCM For the Earth Simulator). As a first step toward high resolution numerical simulation with realistic physical processes, we investigate unstable modes on the condition of super-rotation by nonlinear numerical simulation with simplified physical processes. At initial state zonal super-rotation is assumed to exist. We use the relaxation forcing of the meridional temperature gradient to maintain the zonal flow. In the time evolution of this experimental setting, baroclinic modes grow in the cloud layer with small static stability. The structures of unstable modes are similar to those obtained in the linear stability analysis. We discuss resolution dependency of the results.