  Considering the $\mu$-$\tau$ symmetry, we discuss a direct linkage between
phases of flavor neutrino masses and leptonic CP violation by determining three
eigenvectors associated with ${\rm\bf M}=M^\dagger_\nu M_\nu$ for a complex
flavor neutrino mass matrix $M_\nu$ in the flavor basis. Since the Dirac CP
violation is absent in the $\mu$-$\tau$ symmetric limit, leptonic CP violation
is sensitive to the $\mu$-$\tau$ symmetry breaking, whose effect can be
evaluated by perturbation. It is found that the Dirac phase ($\delta$) arises
from the $\mu$-$\tau$ symmetry breaking part of ${\rm\bf M}_{e\mu,e\tau}$ and
an additional phase ($\rho$) is associated with the $\mu$-$\tau$ symmetric part
of ${\rm\bf M}_{e\mu,e\tau}$, where ${\rm\bf M}_{ij}$ stands for an $ij$ matrix
element ($i,j$=$e,\mu,\tau$). The phase $\rho$ is redundant and can be removed
but leaves its effect in the Dirac CP violation characterized by $\sin (\delta
+ \rho)$. The perturbative results suggest the exact formula of mixing
parameters including that of $\delta$ and $\rho$, which turns out to be free
from the effects of the redundant phases. As a result, it is generally shown
that the maximal atmospheric neutrino mixing necessarily accompanies either
$\sin\theta_{13}=0$ or $\cos(\delta+\rho)=0$, the latter of which indicates
maximal CP violation, where $\theta_{13}$ is the $\nu_e$-$\nu_\tau$ mixing
angle.
