Friction Stir Welding (FSW) has special properties for its low distortion, low residual stress of the joints and needless skilled worker. Application of three-dimensional FSW can apply many industry, etc automobile industry or airplane industry. However, FSW is only applicable to use linier welding at present. To realize three-dimensional FSW, it is most important to improve operation accuracy. On the other hand, it is also important to clarify "the allowance of operation" for application. In this work (A) the influence of deviation; the mismatching between the butt line and the center of tool traveling line, and (B) the influence of gap; the spacing between two plates at butt line, on tensile properties of FSW joints are investigated. A5083-O alloy plates with 3 mm thickness were welded in various deviation or gap spacing conditions by a gate type FSW machine with sufficient rigidity and accuracy. It was found that the allowance of deviation is a half of probe diameter of FSW tool in the case of the butt line was set on the advancing side of tool rotation. So we have the one-fourth of probe diameter allowance in both sides by setting the butt line from the probe center in a quarter of the probe diameter to advancing side. Beyond this allowance range, tensile properties rapidly decreased. Contrary to this, the gap spacing within 2 mm did not affect the tensile properties in this experiment, and the joint strength and ductility over 2 mm gap showed a gradual decrease.