  We performed simulations of collapsars with different Kerr parameters a=0,
0.5, 0.9, 0.95. It is shown that a more rapidly rotating black hole is driving
a more energetic jet. No jet is seen for the case of Schwartzschild black hole
case, while the total energy of the jet is as large as 10^50 erg for a rapidly
rotating Kerr black hole case (a=0.95). In order to explain the high luminosity
of a GRB, it is concluded that a rapidly rotating black hole is favored
('faster is better'). We also find in the case of a=0.95 that (i) the
stagnation region is clearly found in the jet region, (ii) the ordered poloidal
field lines are seen in the jet, (iii) the jet region is surrounded by a
'Wall-like' structure that has a higher pressure than the jet region and
contains strong vertical magnetic fields, and (iv) the jet is initiated by
outgoing Poynting flux from the outer horizon of the black hole
(Blandford-Znajek effect). The bulk Lorentz factor of the jet is still of the
order of unity. However, energy density of electro-magnetic fields dominates
the one of rest-mass in the jet. It can be expected that a relativistic jet
will be seen if we perform a simulation for a longer time scale (of the order
of 10-100 sec).
