Abstract: The invention relates to a device for treating rain water that comprises a housing (4) adapted to be vertically inserted into a dump-separator (1), said dump-separator (1) including a water inlet (2) and a water outlet (12), characterized in that a portion of the side wall of the housing (4) is at least partially curved, and in that said portion includes a filter (5), the inside of the housing (4) being formed by first (6) and second (8) chambers separated by a vertical wall (9) but communicating with each other via a passage (15) formed in the lower portion of the housing (4), said filter (5) defining the outer side wall of the first chamber (6), and the second chamber (8) including an outlet opening adapted for direct communication, via a discharge pipe (7), with said water outlet (12) of the dump-separator (1).