Collecting device adapted to be attached to a robotic lawn mower, where the collecting device comprises a magnet adapted to pick up magnetic metal objects, and where the width of the collecting device is less than the width of the robotic lawn mower to which the collecting device is intended to be attached. The advantage of the invention is that various magnetic metal objects can be picked up from a lawn in an easy and reliable way. The application also includes a robotic lawn mower provided with a collecting device and a method for collecting metal objects on a lawn.
