We carried out global three-dimensional magnetohydrodynamic (MHD) simulations of black hole accretion disks by implementing a three-dimensional resistive MHD code based on the HLLD scheme. The initial state is a torus threaded by weak magnetic fields. The disk magnetic fields amplified by the magneto-rotational instability buoyantly escapes from the disk as the magnetic energy approaches the thermal energy. The direction of the mean magnetic fields inside the disk changes quasi-periodically with period about 10 rational period of the disk. Such disk dynamo can be the origin of low-frequency quasi-periodic oscillations (QPOs) observed in black hole candidates.