  We observe evidence for $D^0 - \bar{D}^0$ mixing by measuring the difference
in apparent lifetime when a $D^0$ meson decays to the CP eigenstates $K^+K^-$
and $\pi^+\pi^-$, and when it decays to the final state $K^-\pi^+$. We find
$y_{CP}=(1.31\pm 0.32({\rm stat.})\pm 0.25({\rm syst.}))%$, 3.2 standard
deviations from zero. We also search for a CP asymmetry between $D^0$ and
$\bar{D}^0$ decays; no evidence for CP violation is found. These results are
based on 540 fb$^{-1}$ of data recorded by the Belle detector at the KEKB
$e^+e^-$ collider.
