Separator for removing impurities, especially smaller particles, from rain water. It comprises a first reservoir (2) and a second reservoir (6). The first reservoir (2) performs the function of removing the heaviest portions of the impurities, but is also provided with a lamellar device (4), wherein small drops and impurities of the rain water may be combined to larger impurity aggregates. The second reservoir (6) is divided into a first chamber (6a) and a second chamber (6b), said chambers communicating with each other. A tube (5) provided with perforations (5′) is arranged above the lamellar device (4), through which rain water containing impurities is able to flow from the first reservoir (2) and into the first chamber (6a). The first chamber (6a) is provided with a filter (9). In this way, a more efficient purification of rain water than using the known separator is provided.