We have searched for mixing in the D 0-D̄ 0 system by measuring the decay-time distribution of D 0 → K + π - 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 D 0→ K + π - to that for the Cabibbb-favored decay D 0 → K - π +. 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 ± 0.017 -0.016 +0.008)%.