  Relativistic jets in active galactic nuclei, galactic microquasars, and
gamma-ray bursts are widely considered to be magnetohydrodynamically driven by
black hole accretion systems, although conversion mechanism from Poynting into
particle kinetic energy flux is still open. Recent detailed numerical and
analytical studies of global structures of steady, axisymmetric
magnetohydrodynamic (MHD) flows with specific boundary conditions have not
reproduced as rapid an energy conversion as required by observations. In order
to find more suitable boundary conditions, we focus on the flow along a
poloidal magnetic field line just inside the external boundary, without
treating transfield force balance in detail. We find some examples of the
poloidal field structure and corresponding external pressure profile for an
efficient and rapid energy conversion as required by observations, and that the
rapid acceleration requires a rapid decrease of the external pressure above the
accretion disk. We also clarify the differences between the fast magnetosonic
point of the MHD flow and the sonic point of de Laval nozzle.
