A variable area frame as a polygonal frame whose area can be changed, comprising: extendable arms (A, B, C, D) forming each side of said polygonal frame; and a coupling mechanism (15, 16) that couples adjoining two of said extendable arms so that said adjoining two extendable arms can move together, in order to allow adjoining two sides of said polygonal frame to extend and contract together, wherein each of said extendable arms includes a plurality of cross units each formed by pivotally coupling two rigid members, which cross each other to form an X-shape, via a middle coupling shaft, and an end coupling portion (13, 14) that pivotally couples ends of adjoining ones of said cross units, wherein said end coupling portion (13, 14) includes inner end coupling shafts located on an inner side of said polygonal frame and outer end coupling shafts located on an outer side of said polygonal frame, and said coupling mechanism (15, 16) includes at a corner of said polygonal frame a first bent member that pivotally couples said inner end coupling shaft of said extendable arm of one of said adjoining two sides of said polygonal frame and said outer end coupling shaft of said extendable arm of the other side, a second bent member that pivotally couples said outer end coupling shaft of said extendable arm of said one of said adjoining two sides of said polygonal frame and said inner end coupling shaft of said extendable arm of the other side, and a corner coupling shaft that pivotally couples said first and second bent members (15, 16) at their intersection, such that the area of the polygonal frame changes with the extension of the extendable arms (A, B, C, D). extendable arms (A, B, C, D) forming each side of said polygonal frame; and a coupling mechanism (15, 16) that couples adjoining two of said extendable arms so that said adjoining two extendable arms can move together, in order to allow adjoining two sides of said polygonal frame to extend and contract together, wherein each of said extendable arms includes a plurality of cross units each formed by pivotally coupling two rigid members, which cross each other to form an X-shape, via a middle coupling shaft, and an end coupling portion (13, 14) that pivotally couples ends of adjoining ones of said cross units, wherein said end coupling portion (13, 14) includes inner end coupling shafts located on an inner side of said polygonal frame and outer end coupling shafts located on an outer side of said polygonal frame, and said coupling mechanism (15, 16) includes at a corner of said polygonal frame a first bent member that pivotally couples said inner end coupling shaft of said extendable arm of one of said adjoining two sides of said polygonal frame and said outer end coupling shaft of said extendable arm of the other side, a second bent member that pivotally couples said outer end coupling shaft of said extendable arm of said one of said adjoining two sides of said polygonal frame and said inner end coupling shaft of said extendable arm of the other side, and a corner coupling shaft that pivotally couples said first and second bent members (15, 16) at their intersection, such that the area of the polygonal frame changes with the extension of the extendable arms (A, B, C, D).