We report the results of a search for D-0-D-0 mixing in D-0-> K(+)pi(-) decays based on 400 fb(-1) of data accumulated by the Belle detector at KEKB. Both assuming CP conservation and allowing for CP violation, we fit the decay-time distribution for the mixing parameters x(') and y('), as well as for the parameter R-D, the ratio of doubly Cabibbo-suppressed decays to Cabibbo-favored decays. The 95% confidence level region in the (x('2),y(')) plane is obtained using a frequentist method. Assuming CP conservation, we find x('2)< 0.72x10(-3) and -9.9x10(-3)< y(')< 6.8x10(-3) at the 95% confidence level; these are the most stringent constraints on the mixing parameters to date. The no-mixing point (0,0) has a confidence level of 3.9%. Assuming no mixing, we measure R-D=(0.377 +/- 0.008 +/- 0.005)%.