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PROBLEM TO BE SOLVED: To easily form a gap for wiring when top plates are butted by providing an upheaving part for forming the gap between the top plate and an abutment object in a part of the top plate when it is abutted on the abutment object. SOLUTION: A first end part 11 is constituted of a straight line part 11a from a part close to one end till a part close to the other end and hemi-cyclic upheaving parts 11b projected from the both ends of the straight line part 11a toward a direction which is orthogonally crossed with the straight line part 11a. The second end part 12 forms an outward swelling shape by a gentle curve as a whole from one end to the other one. The third and fourth end parts 13 and 14 are a straight line shape at positions for connecting the outer ends of the upheaving parts 11b in the first end part 11 and the side ends of the second end part 12. A bumper member consisting of a comparatively soft base material is fitted to the straight line part 11a of the first end part 11, the third end part 13 and the fourth end part 14 and the other respective parts of the top plate 1 are integrally formed by various kinds of top plate base materials including plywood.