PURPOSE: To pivotally support an overhead structure easily by forming a triangular column with a link chain having a hook and raising/lowering a retractable column by a gear drive mechanism so as to form a link type structure.

CONSTITUTION: A triangular column 10 is formed of three link chains 11 to 13 having C-shaped and deviated C-shaped hooks 80 and 81. Next, these hooks 80 and 81 are formed so that they are moved pivotally in the condition of being fixedly engaged with each other, and the link chains 11 to 13 can be pulled up in nested condition using their matching on struts 50 to 55 and the link chains 11 to 13 having recessed grooves. Then, a gear drive mechanism is engaged with two rack gear trains 110a in the chains 11 to 13, and raises and lowers a retractable column 10. Thus, a highly rigid self-supporting triangular column 10 is formed, and an overhead structure can be supported retractably.