Stability of a baroclinic basic flow in gradient wind balance is examined for a very wide range of parameters. When the Rossby number is sufficiently small, it is found that the baroclinic instability occurs not only in the basic flow in geostrophic regime, but also in that in cyclostrophic one. As the Rossby number increases, the baroclinic instability disappears in the nearly geostrophic regime. However, the unstable mode appears again when the dynamic balance changes into the cyclostrophic one; its growth rate is fairly larger than that of the baroclinic instability in the geostrophic regime. It is shown that the baroclinic instability problem in the Venus cloud layer (45--70 km) is not so deviated from that in the terrestrial troposphere, while the cyclostrophic balance is essential in a situation of the Venus lower atmosphere, where the unstable baroclinic wave is predicted to appear.