  By performing 2.5-dimensional special relativistic radiation
magnetohydrodynamics simulations, we study the super-critical accretion disks
and the outflows launched via the radiation force. We find that the outflow is
accelerated by the radiation flux force, but the radiation drag force prevents
the outflow velocity from increasing. The outflow velocity saturates around
30-40% of the light speed around the rotation axis, since then the flux force
balances with the drag force. Our simulations show that the outflow velocity is
kept nearly constant in the regime of \dot{M}_{BH} ~ 100-1000 L_{Edd}/c^2,
where \dot{M}_{BH} is the mass accretion rate, L_{Edd} is the Eddington
luminosity, and c is the light speed. Such a faster outflow is surrounded by a
slower outflow of ~ 0.1c. This velocity is also determined by force balance
between the radiation flux force and the radiation drag. The radiation drag
works to collimate the slower outflow in cooperation with the Lorentz force,
although the faster outflow is mainly collimated by the Lorentz force. The
kinetic energy is carried by the slower outflow rather than by the faster
outflow. The total kinetic luminosity of the outflow as well as the photon
luminosity is ~ L_{Edd}, almost independent of the mass accretion rate.
