A method of manufacturing a cylindrical body, comprising the step of forming the cylindrical body by bending a plate-like work having first projected finger to fourth projected finger at four corned parts and allowing the end faces thereof to abut on each other, wherein the main surface of the cylindrical body on the side where sags are present is formed in an outer peripheral wall surface and the rear surface thereof on the side where the burrs are present is formed in an inner peripheral wall surface, and a first projected part is formed of the first projected finger and the third projected finger and a second projected part is formed of the second projected finger and the fourth projected finger. After the cylindrical body is held by friction stir welding devices, the probe of a friction stir welding tool is buried from the direction of either of the first projected part and the second projected part, and scanned in the direction of the other of the second projected part and the first projected part. The probe is buried and scanned in the state of being displaced to an advancing side.