A robot arm mechanism is capable of preventing a first piece string from colliding with a support column inner wall in the robot arm mechanism having a linear extension and retraction joint. The robot arm mechanism includes a plurality of first pieces that are connected in a string shape and a plurality of second pieces that are connected in a string shape, a feed mechanism section that supports the first and second strings movably forward and backward, a storage section in a square cylinder shape that stores the first pieces and the second pieces, and a guide that guides reciprocating movement of the first pieces between the feed mechanism section and the storage section from outside of the first pieces. The guide has a plurality of guide pieces that are connected bendably in mutual front and rear end surfaces to deform in accordance with rising and lowering movement of a rising and lowering rotation joint, and the guide hangs into the storage section from a tip position where the guide is fixed to the feed mechanism section.