A purpose is to prevent a first connection piece string from colliding against a second connection piece string in a robot arm mechanism including a linear extension and retraction joint. In the robot arm mechanism having the linear extension and retraction joint, the linear extension and retraction joint includes an arm section 2, and an ejection section 29 for supporting the arm section 2, the arm section 2 includes a first connection piece string 21 made by a plurality of first connection pieces 23, and a second connection piece string 20 made by a plurality of second connection pieces 22, the second connection piece string 20 is sent out forward from the ejection section 29 together with the first connection piece string 21 in a state where the second connection piece string 20 is joined to the first connection piece string 21, and a flexible guide rail 41 for separating the first connection piece string 21 from the second connection piece string 20 and guiding the second connection piece string 20 to the ejection section 29 is interposed between the first connection piece string 21 and the second connection piece string 20 behind the ejection section 29.