PURPOSE: To increase the rigidity of a mast structure by forming batten members working as cross sash members longeron members working as longitudinal members, and diagonal members arranged diagonally between joints of these members.

CONSTITUTION: An expansive/contractive mast is formed from a plurality of approx. triangular batten members 1, a plurality of longeron members 2 which are to couple together the crests of members 1, and diagonal members 3 each stretching along the quadrangular diagonal line constituted from the members 1 and 2. Then rotary members 21, 22 of each longeron member 2 and turned to be arranged in a straight line form, and the members 2 are spread straightly to elongate the mast, while the members 3 are also elongated fully so as to work as diagonal members. Thus the rigidity of the mast is increased. Then, the pivoting centers of pins 23, 24 of the member 2 overshoot the center in the amount Oc and are locked. This can increase the rigidity of the mast structure.