PURPOSE: To facilitate coupling of an outdoor trench connected to other building, by previously installing a projecting part, having a wall thickness being thinner that of an outer wall, at the outer wall of the building.

CONSTITUTION: An opening 3 is previously formed in an outer wall 1 of a building, and is covered with a shutter 4 such as iron plate. The outside of the building part, wherein the opening is formed, is covered with a "ko"(Japanese:katakana,2533)-shaped projection part 5 having a wall thickness being sufficiently thinner than that of the outer wall 1. The level of the floor surface of a point of the projection part 5 lowers the further outward from the building this point is situated, and a sump pit 7 equipped with a sump pump 6 is formed in a lowermost position. The connection of an outdoor trench 2 is such that, after the forward end of the projection part 5 is knocked down, the end of the outdoor trench 2 being open to the building 8 is connected to the knocked down part.