The primary objective of this study is to compare the incidence of hemorrhagic events in patients treated for non-valvular atrial fibrillation with DU-176b at each dose level versus warfarin potassium (warfarin). The secondary objective includes between-group comparisons with regard to incidence of thromboembolic events, pharmacodynamic parameters, and biomarkers for the efficacy evaluation, as well as incidence of adverse events and adverse reaction for the safety evaluation.