An analysis of plasma initiation by neutral beams in the presence of a confining vacuum magnetic field is presented. Plasma startup by NBI alone was demonstrated in LHD in 1999, and since then this has become an indispensable technique for producing plasmas over a wide range of magnetic field strengths. The method is simple: injecting high power neutral beams into a thin gas target, waiting for a few hundred milliseconds until a thin `seed' plasma appears and then adding a gas puff or injecting a pellet to build up the main plasma. The mechanism of the process is considered to be as follows. A small number of fast ions created by collisions with background neutrals are confined in the magnetic field. They ionize the background neutrals and heat the thin seed plasma produced earlier. Numerical calculations based on this physical model can reproduce well the observed time evolution and input power dependence of the plasma buildup. The essential point is that its electron temperature should be high enough to ionize the neutrals added by gas puffing or pellet injection. Because the main energy loss process from this partially ionized plasma is by charge exchange of ions, the ionization degree should become high enough that the electron temperature increases. Therefore, the initial target gas density is important for building up plasma within a limited time.