  The magnetic properties of the antiferromagnetic spin ladder with the
next-nearest neighbor interaction, particularly under external field, are
investigated by the exact diagonalization of the finite clusters and size
scaling techniques. It is found that there exist two phases, the rung-dimer and
rung-triplet phases, not only in the nonmagnetic ground state but also
magnetized one, where the phase boundary has a small magnetization dependence.
Only in the former phase, the magnetization curve is revealed to have a
possible plateau at half the saturation moment, with a sufficient frustration.
