  Significant fraction of matter in supercritical (or super-Eddington)
accretion flow is blown away by radiation force, thus forming outflows,
however, the properties of such radiation-driven outflows have been poorly
understood. We have performed global two-dimensional
radiaion-magnetohydrodynamic simulations of supercritical accretion flow onto a
black hole with 10 or 10^8 solar masses in a large simulation box of 514 r_S x
514 r_S (with r_S being the Schwarzschild radius). We confirm that uncollimated
outflows with velocities of 10 percents of the speed of light emerge from the
innermost part of the accretion flow over wide angles of 10 - 50 degree from
the disk rotation axis. Importantly, the outflows exhibit clumpy structure
above heights of ~ 250 r_S. The typical size of the clumps is ~ 10 r_S, which
corresponds to one optical depth, and their shapes are slightly elongated along
the outflow direction. Since clumps start to form in the layer above which
(upward) radiation force overcomes (downward) gravity force, Rayleigh-Taylor
instability seems to be of primary cause. In addition, a radiation hydrodynamic
instability, which arises when radiation funnels through radiation-pressure
supported atmosphere, may also help forming clumps of one optical depth.
Magnetic photon bubble instability seems not to be essential, since similar
clumpy outflow structure is obtained in non-magnetic radiation-hydrodynamic
simulations. Since the spatial covering factor of the clumps is estimated to be
~ 0.3 and since they are marginally optically thick, they will explain at least
some of rapid light variations of active galactic nuclei. We further discuss a
possibility of producing broad-line clouds by the clumpy outflow.
