Joint coupling links of robot arm to each other includes: plurality of plate-shaped segments lined up in plate thickness direction; and operating wires extending through plurality of segments in plate thickness direction. Groove extending in second direction and having partially circular cross section is provided at first-direction middle portion of main surface out of two main surfaces of adjacent segments, two main surfaces facing each other in plate thickness direction. Projection extending in second direction and having partially circular cross section is provided at first-direction middle portion of main surface. Curvature of projection is larger than curvature of groove. Projection is fitted in groove, and bottom portion of groove and top portion of projection contact each other. Dimension of projection in plate thickness direction is larger than dimension of groove in plate thickness direction and smaller than length of line connecting bottom portion of groove and edge portion of groove.