A shaft (3) has a central axis (L3) and includes first and second members (11, 12) that are identical in shape and that form a shaft portion for providing one end of the shaft (3) in an axial direction that the central axis extends in. Each of the first and second members (11, 12) includes a plate (111, 121) and an arm portion (112, 122). The plates (111, 121) extend in the axial direction. The arm portions (112, 122) are located closer to the end of the shaft (3) than the plates (111, 121) and are located farther from the central axis (L3) than the plates (111, 121). The plates (111, 121) face each other across the central axis (L3). The arm portions (112, 122) are couplable with a joint spider.