  Employing a method of exact diagonalization for finite-size systems, we
investigate the magnetization curve in the ground state of an antiferromagnetic
spin-1/2 ladder with additional exchange interactions on diagonal bonds, which
is equivalent to an antiferromagnetic spin-1/2 chain with bond-alternating
nearest-neighbor and uniform next-nearest-neighbor interactions. It is found
that a half-plateau appears in the magnetization curve in a certain range of
the interaction constants. This result is discussed in connection with the
necessary condition for the appearance of the plateau, recently given by
Oshikawa et al.
