[en] The linear stability of a magnetized gas layer is studied for accretion disks and galactic gas disks. The gas layer is assumed to be located at some distance from a point mass which is the origin of gravity. The magnetic fields are assumed to run parallel to the disk in a magnetohydrostatic equilibrium state. In this equilibrium, the sound speed and the Alfven speed are taken to be uniform. The medium is an ideal gas and perfectly conducting. The eigenfunctions and the growth rates are computed for isothermal perturbations. It is found that the most unstable mode breaks the inversion symmetry of the disk. In the perturbed state, some magnetic field lines run across the original disk plane, and the gas disk is warped. The two-dimensional configuration of the disk in the perturbed state is displayed graphically. In the unstable modes, the disturbance has a large amplitude in the region where the gravitational acceleration is large. In cold and-or magnetic pressure dominated disks, the minimum growth time is 2-5H-v_A, where v_A is the Alfved speed and H is the pressure scale height defined where the gravitational acceleration is maximum. (author)