PROBLEM TO BE SOLVED: To improve the degree of freedom in respect to changing the shape, position, size, and the like, of a hole 5 which leads water that has passed through a peeling gap 3 into the inner periphery of a drainage basin 1.SOLUTION: A drainage basin 1 comprises upper and lower cylindrical bodies 10 and 11 installed as separate bodies, and fixed at their relative positions by fasteners 12. Then, an annular gutter space 4 is formed by fitting a lower end part 10d of the upper cylindrical body 10 into an opening 8du of the lower cylindrical body 11. A hole 5 is formed between the lower end part 10d and a middle section 11b of the lower cylindrical body 11, and the gutter space 4 is opened to the inner periphery of the lower cylindrical body 11 by the hole 5. Thus, the shape, position, size, and the like, of the hole 5 can be freely changed by changing the relative position between the upper and lower cylindrical bodies 10 and 11, or by changing the shape of the inner wall of the lower end part 10d and middle section 11b. Therefore, the degree of freedom in respect to changing the shape, position, size, and the like, of the hole 5 can be improved.SELECTED DRAWING: Figure 1
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