  Employing mainly numerical methods, we explore the ground-state phase diagram
of an anisotropic $S=1/2$ ladder, in which leg interactions are uniform and
isotropic, while rung interactions are alternating and have a common Ising-type
anisotropy. We determine the phase diagram in the case where $J_{\rm leg}=0.2$
(antiferromagnetic), $J_{\rm rung}=-1.0$ (ferromagnetic) and $|J_{\rm
rung}'|\!\leq\!1.0$, the first one being the magnitude of the leg interaction
and the second and third ones those of the rung interactions, which are
alternating. It is emphasized that the system has a frustration when $J_{\rm
rung}'$ is positive. We find that, in the frustrated region, the Haldane state
appears as the ground state even when the Ising character of rung interactions
is strong. This appearance of the Haldane phase is contrary to the ordinary
situation, and it is called the inversion phenomenon concerning the interaction
anisotropy. We also find that an incommensurate state becomes the ground state
in a portion of the Haldane phase region.
