A joint of a robot arm, the joint coupling links of the robot arm to each other, the joint comprising: a plurality of plate-shaped segments lined up in a plate thickness direction, each of the plurality of segments having a first-direction middle portion positioned at a middle of the segment in a first direction perpendicular to the plate thickness direction; and an operating wire (71a,71b) extending through the plurality of segments in the plate thickness direction at both sides of the first-direction middle portion of each of the plurality of segments, the sides being opposed to each other in the first direction, wherein: the plurality of plate-shaped segments includes a first segment and a second segment being adjacent to each other, a groove having a partially circular cross section and extending in a second direction perpendicular to the first direction and the plate thickness direction is provided at a first surface of the first-direction middle portion of the first segment; a projection having a partially circular cross section and extending in the second direction is provided at a second surface of the first-direction middle portion of the second segment, the first surface and the second surface are facing each other in the plate-thickness direction, the projection is fitted in the groove; and a dimension of the projection in the plate thickness direction is larger than a dimension of the groove in the plate thickness direction and smaller than a length of a line connecting the bottom portion of the groove and an edge portion of the groove; characterized in that a curvature of a cross-sectional shape of the projection is larger than a curvature of a cross-sectional shape of the groove, and a bottom portion of the groove and a top portion of the projection contact each other. a plurality of plate-shaped segments lined up in a plate thickness direction, each of the plurality of segments having a first-direction middle portion positioned at a middle of the segment in a first direction perpendicular to the plate thickness direction; and an operating wire (71a,71b) extending through the plurality of segments in the plate thickness direction at both sides of the first-direction middle portion of each of the plurality of segments, the sides being opposed to each other in the first direction, wherein: the plurality of plate-shaped segments includes a first segment and a second segment being adjacent to each other, a groove having a partially circular cross section and extending in a second direction perpendicular to the first direction and the plate thickness direction is provided at a first surface of the first-direction middle portion of the first segment; a projection having a partially circular cross section and extending in the second direction is provided at a second surface of the first-direction middle portion of the second segment, the first surface and the second surface are facing each other in the plate-thickness direction, the projection is fitted in the groove; and a dimension of the projection in the plate thickness direction is larger than a dimension of the groove in the plate thickness direction and smaller than a length of a line connecting the bottom portion of the groove and an edge portion of the groove; characterized in that a curvature of a cross-sectional shape of the projection is larger than a curvature of a cross-sectional shape of the groove, and a bottom portion of the groove and a top portion of the projection contact each other. the plurality of plate-shaped segments includes a first segment and a second segment being adjacent to each other, a groove having a partially circular cross section and extending in a second direction perpendicular to the first direction and the plate thickness direction is provided at a first surface of the first-direction middle portion of the first segment; a projection having a partially circular cross section and extending in the second direction is provided at a second surface of the first-direction middle portion of the second segment, the first surface and the second surface are facing each other in the plate-thickness direction, the projection is fitted in the groove; and a dimension of the projection in the plate thickness direction is larger than a dimension of the groove in the plate thickness direction and smaller than a length of a line connecting the bottom portion of the groove and an edge portion of the groove; characterized in that a curvature of a cross-sectional shape of the projection is larger than a curvature of a cross-sectional shape of the groove, and a bottom portion of the groove and a top portion of the projection contact each other.