  We have performed 2.5-dimensional general relativistic magnetohydrodynamic
(MHD) simulations of the gravitational collapse of a magnetized rotating
massive star as a model of gamma ray bursts (GRBs). This simulation showed the
formation of a disk-like structure and the generation of a jet-like outflow
inside the shock wave launched at the core bounce. We have found the jet is
accelerated by the magnetic pressure and the centrifugal force and is
collimated by the pinching force of the toroidal magnetic field amplified by
the rotation and the effect of geometry of the poloidal magnetic field. The
maximum velocity of the jet is mildly relativistic ($\sim$ 0.3 c).
