A welding system of cylindrical structures configured to weld a welding end surface of an upper cylindrical structure and a welding end surface of a lower cylindrical structure, each of which being an end surface in an axial direction thereof and facing each other, the welding system comprising: two or more welding apparatuses (170, 172, 174, 176) which are opposite to the welding end surfaces (120, 122) of the upper cylindrical structure and the lower cylindrical structure and are disposed at equal arrangement intervals in the circumferential direction of the cylindrical structures (92, 93); a moving device which is configured to rotate the upper cylindrical structure and the lower cylindrical structure relative to the welding apparatuses (170, 172, 174, 176) in a circumferential direction of the cylindrical structures (92, 93); a control device which is configured to control operations of the welding apparatuses (170, 172, 174, 176) and the moving device, wherein each of the welding apparatuses (170, 172, 174, 176) has a filler metal and a heating source which melts the filler metal, and is configured to melt and fuse the filler metal on the welding end surfaces (120, 122) to thereby weld the welding end surfaces (120, 122), and the control device is further configured to control the moving device to continuously rotate the upper cylindrical structure and the lower cylindrical structure by an angle of the arrangement interval of the welding apparatuses (170, 172, 174, 176), while welding the welding end surfaces (120, 122) with the welding apparatuses (170, 172, 174, 176); the welding system being characterised by a side surface position detection unit which is configured to measure a position of a side surface of the upper cylindrical structure; an upper surface position detection unit which is configured to measure a position of an upper surface of the upper cylindrical structure; and a width detection unit which is configured to measure displacement between the upper cylindrical structure and the lower cylindrical structure, and the control device is further configured to detect a positional deviation of the upper cylindrical structure with respect to the lower cylindrical structure based on the position detected by the side surface position detection unit, the position detected by the upper surface position detection unit and the displacement detected by the width detection unit, and control the operations of the moving device and the welding apparatuses (170, 172, 174, 176) based on the detected positional deviation. two or more welding apparatuses (170, 172, 174, 176) which are opposite to the welding end surfaces (120, 122) of the upper cylindrical structure and the lower cylindrical structure and are disposed at equal arrangement intervals in the circumferential direction of the cylindrical structures (92, 93); a moving device which is configured to rotate the upper cylindrical structure and the lower cylindrical structure relative to the welding apparatuses (170, 172, 174, 176) in a circumferential direction of the cylindrical structures (92, 93); a control device which is configured to control operations of the welding apparatuses (170, 172, 174, 176) and the moving device, wherein each of the welding apparatuses (170, 172, 174, 176) has a filler metal and a heating source which melts the filler metal, and is configured to melt and fuse the filler metal on the welding end surfaces (120, 122) to thereby weld the welding end surfaces (120, 122), and the control device is further configured to control the moving device to continuously rotate the upper cylindrical structure and the lower cylindrical structure by an angle of the arrangement interval of the welding apparatuses (170, 172, 174, 176), while welding the welding end surfaces (120, 122) with the welding apparatuses (170, 172, 174, 176); the welding system being characterised by a side surface position detection unit which is configured to measure a position of a side surface of the upper cylindrical structure; an upper surface position detection unit which is configured to measure a position of an upper surface of the upper cylindrical structure; and a width detection unit which is configured to measure displacement between the upper cylindrical structure and the lower cylindrical structure, and the control device is further configured to detect a positional deviation of the upper cylindrical structure with respect to the lower cylindrical structure based on the position detected by the side surface position detection unit, the position detected by the upper surface position detection unit and the displacement detected by the width detection unit, and control the operations of the moving device and the welding apparatuses (170, 172, 174, 176) based on the detected positional deviation.