  The magnetization process of the S=1/2 antiferromagnetic spin ladder at T=0
is studied by the exact diagonalization of finite clusters and size scaling
analyses. It is found that a magnetization plateau appears at half the
saturation value (m=1/2) in the presence of a sufficiently large
next-nearest-neighbor exchange interaction to yield the frustration, when the
rung coupling is larger than the leg one. The phase diagram at m=1/2 is given
by the analysis based on the conformal invariance. The magnetization curves are
also presented in several cases.
