  We have searched for mixing in the D0-D0bar system by measuring the
decay-time distribution of D0->K+pi- decays. The analysis uses 90 fb^{-1} of
data collected by the Belle detector at the KEKB e+e- collider. We fit the
decay-time distribution for the mixing parameters x' and y' and also for the
parameter R_D, which is the ratio of the rate for the doubly-Cabibbo-suppressed
decay D0->K+pi- to that for the Cabibbo-favored decay D0->K-pi+. We do these
fits both assuming CP conservation and allowing for CP violation. We use a
frequentist method to obtain a 95% C.L. region in the x'^2 - y' plane. Assuming
no mixing, we measure R_D = (0.381 \pm 0.017 +0.008 -0.016) %.
