  We study the magnetization process of the $S=1/2$ antiferromagnetic spin
ladder in the presence of the second and the third-neighbor couplings which
lead to frustration with the typical nearest-neighbor coupling. We use
degenerate perturbation theory and level spectroscopy analysis of the numerical
diagonalization data of the Hamiltonian for finite systems. We find two kinds
of plateaux at half the saturation moment in the magnetization curve. One is
mainly due to the second-neighbor couplings and the other to the third-neighbor
couplings. The mechanisms of these two plateaux are quite different with each
other.
