| 摘要 |
The section of a main pipe wall made of synthetic resin material is formed to be substantially triangular and one inclined wall approaches the other inclined wall while the inclined wall goes over a center line passing through a top of both the inclined walls forming the main pipe wall and the approaching posture is capable of being maintained by itself. A second pipe wall, the wall thickness of which is smaller than that of the main pipe wall, is integrally connected with the inside of the pipe main wall, the section of the second pipe wall is substantially triangular, a bottom of the second pipe wall and predetermined portions of both the inclined walls at the bottom side adhere to a bottom of the main pipe wall and both the inclined walls, and a small space is formed between a top of the second pipe wall and the top of the main pipe wall.
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