Two gaseous disks having different dispersion velocities are assumed to simulate the disk-shaped galaxy which contains both stars and gas. An analytical formula has been obtained for the criterion of local Jeans gravitational instability of the whole system against axisymmetric disturbances. It is shown that the stability of the system is determined by the density ratio and by the dispersion velocities of the two disks. The contributions of the dispersion velocities to the stability compensate each other for a fixed density ratio.