to fixedly mount the corresponding panel body 10 thereto these small halls 22 e may be formed at three or five or more portions of each mounting plate 22 c on the circumference of each rolling section 22 a is formed a pinion that is to be meshed with the corresponding rack of the movable piece 21 a through hole is formed along the center axis of the rolling section 22 a when the movable piece 21 has a trapezoidal shape the rolling section 22 a has a conical trapezoidal shape like a bevel gear the connection element 24 is to connect the rolling sections 22 a together while allowing the same to roll and has intermediate portions respectively contacting the opposite contacting surfaces 21 b of the movable piece 21 a pair of connection plates 24 a each having opposite ends protruding outwards from the opposite side surfaces 21 a of the movable piece 21 each plate having a running track shape and pins 24 b inserted in the through holes of the rolling sections 22 a through the opposite ends of the connection plates 24 a thus the connection plates 24 a each have a length longer than the distance between the pair of the rolling sections 22 a that hold the movable piece 21 therebetween for the pins 24 b that extend through the through holes of the rolling sections 22 a two pins may be provided for each side of the connection plates 24 a so that they are only inserted up to portions inward from the corresponding ends of the through holes of the rolling.