Collimated outflows (or jets) with relativistic speeds are driven in various celestial objects. The origin of relativistic jets are thought to be black holes with their accretion flows, although the detailed mechanism remains unknown. A widely discussed model is that rotational energy of the accretion flow is magnetohydrodynamically converted into the outflow energy in analogy of pulsar winds. One of the major problems for this model is how Poynting flux can be efficiently transferred to particle kinetic energy flux. Recently some numerical simulations of global structure of steady magnetohydrodynamic jets have been significantly developed, but they have not succeeded in reproducing the energy conversion as efficient as suggested by observations. We focus on the motion along a magnetic field line and show some examples of 2-dimensional field configuration for which the energy conversion is efficient enough, also discussing the conditions required for the external boundary of the jet.