To solve the problem with conventional spring-winding types which cause a breakdown or insufficient winding force.

Both sides of sheet 4 and 5 which are connected with both sides of a sliding board 1 are wound around a pair of rotary winding drums 6 and 7, and an endless chain 14 is engaged with sprocket wheels 12 and 13 which are mounted on the winding drums 6 and 7. When the sliding board 1 is moved, also one side of the sheet 4 is moved, and the sheet 4 and 5 are stretched by a winding force on the winding drum 7, which is caused by a winding force on the other winding drum 6 of the sheet 4, through the chain 14. Besides tensioning parts 17 and 18 which provide tensioning force and can be expanded/contracted are fit respectively in linear sections 15 and 16 of the chin 14, and a larger tensioning force is applied on the main-drive tensioning part 17, in order to give a winding force onto the winding drums 6 and 7, consequently to stretch the both sides sheet 4 and 5 without looseness.