  We present a measurement of the $D^0$-$\overline{D}^0$ mixing parameter
$y_{CP}$ using a flavor-untagged sample of $D^0\to K_S^0K^+K^-$ decays. The
measurement is based on a 673 fb$^{-1}$ data sample recorded by the Belle
detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We find $y_{CP} =
(0.21 \pm 0.63 ({\rm stat.})\pm 0.78 (\rm syst.) \pm 0.01(\rm model))%$.
