PURPOSE: To simplify a work to position an arm by forming an oblong hole for positioning formed in a curved shape in the shape of a concentric circle centering around a joint and in the one of an arm and a base part to which the joint is supported and removably inserting a pair of arm positioning members, one of which is brought into slide contact with the oblong hole and another one of which a guide roller is mounted, in the other.

CONSTITUTION: By controlling movement through drive of a joint by means of an arm 4 in a state that the one arm positioning member 14 to which a guide roller 14a is arranged is inserted in an oblong hole 10, the one arm positioning member 14 is moved to the one end of the oblong hole 10 and brought into contact therewith to perform positioning in the direction of the one end. Thereafter, by inserting the other positioning member 15 in the oblong hole 10, the other positioning member is brought into contact with the other end of the oblong hole 10 to perform positioning in the direction of the other end. This constitution facilitates positioning of the arm 4 in a folding containing position, performs reliable positioning, and simplifies a positioning work as much as possible.