The present invention relates to a device (1) for filtering storm water inside a storm drain. The device (1) comprises a filter unit (2), a metal capturing ballast unit (6), and a floating carrier (4) for carrying the filter unit and the metal capturing ballast unit (6). Whereby, in use, the device (1) is arranged in a floating condition in the storm water of the storm drain. The filter unit (2) is at least partially arranged above a surface (5) of the storm water, and the metal capturing ballast unit (6) is arranged below the surface (5) of the storm water. The metal capturing ballast unit (6) comprises a solid material for capturing metal and providing ballast.