In a joint structure for coupling a first component (21) to a second component (22) in a joint portion of a fluid flow passage, the second component (22) has a portion thereof inserted into the first component (21) to engage with the first component (21). The second component (22) is provided with a flange portion (26), which is disposed adjacent to a bottom surface of the first component (21) and welded or bonded to the bottom surface, so that these components (22, 21) are coupled with each other in an easy and a fluid-tight manner in a condition in which they are prevented from rotating relative to each other without using any locking pin. There is no fear that the components (22, 21) dissolve in a fluid or gaseous material passing through the fluid flow passage. The joint structure is easy in maintenance.