The gravitational instabilities are investigated for rigidly rotating isothermal gaseous disks with a magnetic field whose mean direction is parallel (case A) and perpendicular (case B) to the rotation axis. The linearized equation for the perturbation becomes an eigen value problem which is solved numerically. The stability depends on two non-dimensional parameters p2 ≡ B20/(4π2Gσ20) and q2 ≡ Ω20/(π Gρ0c) where B0, σ0, Ω0 and ρ0c are the magnetic field strength, the column density of the disk, the uniform angular velocity and the central density, respectively.