A robot arm (50) of the present invention includes, as a direct acting extensible/retractable joint (J3), an arm section (2) constituted by an upper structure group (20) and a lower structure group (21). Groups (20) and (21), having an arrangement in which structures are connected in series, partially engage so as to form a direct rigid combined structure, and are separated so as to release the rigid structure. An arm length can be adjusted arbitrarily. Section (2) can have a plane surface having no gap to prevent entry of a finger/dust. Separation of Groups (20) and (21) allows an upper structure (22) and a lower structure (23) to rotate around their rotational axes, to realize compact storage inside a robot arm supporting member (1). It is thus possible to prevent significantly entry of a finger/dust, and provide a compact direct acting extensible and retractable arm mechanism.