To select the welding conditions that can ensure a particular fatigue strength for safe use in a structure produced by the FSW (Friction Stir Welding) of Mg alloys, this paper proposes a method for determining the range of welding conditions that can ensure a particular joint defect size limit. The welding parameters used are the rotation speed of the tool ω and the welding speed v. Generally, a welding condition is selected using only the fatigue strength as an index. However, ensuring the fatigue strength in a particular welding condition does not insure the strength and reliability of a welded part, because actual welding is not typically carried out in a particular welding condition. Therefore, a range of conditions that can ensure a particular fatigue strength is required. Moreover, we also consider parameters that represent the factors that decrease fatigue strength, namely, √area, HV, and R, because the welding conditions that can ensure a particular fatigue strength are determined by an evaluation of these factors, as well. Therefore, in this study, a method for determining a range of welding conditions that can ensure a welding defect size limit is proposed. Furthermore, FSW welding tests were carried out and the validity of the proposed method was evaluated. X-ray transmission measurements were carried out to measure the welding defect size obtained using the method. Then, the prediction of the maximum welding defect size was carried out for a welding length of 25 [m], which is representative of body of a the Shinkansen rolling stock car.