To provide a friction stir welding device capable of reducing non-joined part at both ends in the longitudinal direction when butt-welding the side surface parts of long planar members against each other.

A friction stir welding device is provided with a processing tool 6a, a pressurizing roller 7a disposed before the processing tool in the moving direction for pressurizing the abutting part 3a of a long planar member from an upper surface side, an auxiliary roller 7b disposed before the pressurizing roller in the moving direction for moving integrally with the pressurizing roller, a pressing jig 4 for pressing the long planar member abutted at both ends in the longitudinal direction of the long planar member altogether from the upper surface side, and a running-on jig 5 disposed near the pressing jig. The processing tool, the pressurizing roller and the auxiliary roller are relatively moved integrally to the abutting part, and when the processing tool approaches to the pressing jig, the auxiliary roller gets on the running-on jig and moves on the running-on jig while pressurizing and the pressurizing roller gets on the running-on jig and moves on the running-on jig while pressurizing.