  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 with the Belle
detector at the KEKB asymmetric-energy $e^+ e^-$ collider. Using a method based
on measuring the mean decay time for different $K^+K^-$ invariant mass
intervals, we find $y_{CP} = (+0.11\pm 0.61 ({\rm stat.})\pm 0.52 (\rm
syst.))%$.
