The purpose of the present invention is to improve the mobility of a linearly moving extendable mechanism. A robot arm mechanism according to the present invention has rotational joints and a linearly moving extendable joint. The linearly moving extendable joint has: a plurality of first coupling pieces (23) on the bottom plate side thereof, which are bendably coupled and have a u-shaped cross section; and a plurality of second coupling pieces (22) which are bendably coupled and have a plate shape. The second coupling piece located at the leading end among the plurality of second coupling pieces is connected to the first coupling piece located at the leading end among the plurality of first coupling pieces. The first and second coupling pieces are joined, thereby restraining bending and constituting a pillar body. The pillar body is released by separating the first and second coupling pieces. An exit portion (32) forms the pillar body by joining the first and second coupling pieces and supports the pillar body. A linear gear (30) is formed on the bottom plate surface of each of the first coupling pieces. Inside the exit portion, a drive gear (29) meshes with the linear gears of the first coupling pieces joined to the second coupling pieces.