To prevent the damage of a distributing pipe even at the time of a large quantity of precipitation at comparatively low cost, and to maintain drainage capacity by a pipeline by installing an air vent device, from which air is not flowed back, on the upper side of a bend.

An air vent device is composed of a top plate 9 in the bend of an underground drain pipe, a box mounted on the upper side of the top plate 9 and made of concrete, a pressure pipe communicating both top plate 9 and box, and a combustion hole 4, etc., drying clay 10 packed into the box made of concrete. When a pipeline is filled with drain by a large quantity of precipitation, air remaining in a bend section is discharged from the air vent device to the outside of the pipeline. Consequently, since an air reservoir is removed in the pipeline, water is drained by a total cross section. The pipeline is worked as a siphon at that time, and drainage discharge is increased suddenly. Accordingly, even when there is a large quantity of precipitation, a back flow and the damage of the pipeline are not generated by the air reservoir, and drainage capacity under the state of a total sectional flow can be maintained.