Coupling segment, direct-acting extension/retraction mechanism, and robot arm mechanism

This direct-acting extension/retraction mechanism is provided with: a first coupling-segment row (21) comprising a plurality of first coupling segments (23); a second coupling-segment row (22) comprising a plurality of second coupling segments (24); a linear gear (239) comprising a plurality of teeth (240) provided to the rear surface of each of the first coupling segments (23); an ejection part (30) for supporting a columnar body formed by joining the first and second coupling-segment rows (21, 22); and a drive gear (50) which meshes with the linear gear (239). The front and rear teeth (240f, 240e) on the first coupling-segment row (21) which are disposed next to each other with coupled sections therebetween are provided so as to not overlap each other when viewed from the direction of the central axis of the columnar body.