PURPOSE: To stabilize structure and also prevent the sagging of a film material by pin-joining large and small beams composed of a wooden laminated lumber, spreadingly providing the film material to the part surrounded with individual beam materials, and spreadingly providing pipe arches in the back of the film material.

CONSTITUTION: Large and small beams 2 and 3 composed of a wooden laminated lumber are mounted in a parallel crosses state by pin joining, and horizontal beams 4 with a laminated lumber is provided across between the lower end parts of individual beams 2 and 3. Then film materials 5 of a rectangle or triangle state are spreadingly provided with individual parts surrounded with individual beams 2 and 3 as boundaries. Moreover steel-made pipe arches 6 are spreadingly provided in the back of the materials 5 in the shape lengthwise halving a film surface, in-plane tension is given to the materials 5, and sagging due to creep is prevented.