  The frustrated quantum spin system on the distorted diamond chain lattice is
investigated using the numerical diagonalization of finite-size clusters and
the level spectroscopy analysis. In the previous work this system was revealed
to exhibit the 1/3 magnetization plateau due to two different mechanisms
depending on the coupling parameters, and the phase diagram at the 1/3
magnetization was obtained. In the present work it is found that the 1/3
magnetization plateau vanishes for sufficiently large $XY$-like coupling
anisotropy. The phase diagram based on the level spectroscopy analysis is also
presented.
