Abstract: An upper end of an outer pipe 16 is connected to a gutter, and a lower end of the outer pipe is connected to a water collecting box communicating with a sewer pipe. An inner pipe 18 which is arranged apart from an inner circumferential surface of a vertical portion 16d of the outer pipe at an outer circumferential edge of an upper portion thereof is connected to a storage-percolating box at a lower end thereof. A ring-like flow-separating piece 24 is disposed on an inner circumferential surface of the vertical portion of the outer pipe above an outer end of the inner pipe by a predetermined distance.