A joint 25 coupling links 22 of a robot arm 2 to each other includes: a plurality of plate-shaped segments 4 lined up in a plate thickness direction Z； and operating wires 71a and 71b extending through the plurality of segments 4 in the plate thickness direction Z. A groove 43 extending in a second direction X and having a partially circular cross section is provided at a first-direction-Y middle portion of a main surface 41 out of two main surfaces 41 and 42 of the adjacent segments 4, the two main surfaces 41 and 42 facing each other in the plate thickness direction Z. A projection 48 extending in the second direction X and having a partially circular cross section is provided at a first-direction-Y middle portion of the main surface 42. A curvature of the projection 48 is larger than a curvature of the groove 43. The projection 48 is fitted in the groove 43, and a bottom portion 43a of the groove 43 and a top portion 48a of the projection 48 contact each other. A dimension of the projection 48 in the plate thickness direction Z is larger than a dimension of the groove 43 in the plate thickness direction Z and smaller than a length of a line L connecting the bottom portion 43a of the groove 43 and an edge portion 43b of the groove 43.
