A device for attaching a guide unit to a first furniture part which is movably guided relative to a second furniture part, wherein the guide unit includes a basic unit rail disposed on the second furniture part and a motion rail disposed on the first furniture part. Two bearing portions are disposed spaced apart on a lower edge of the device, which is provided for disposal on the motion rail. Each of the bearing portions is configured on a side which faces the respective other bearing portion, such that the motion rail can be disposed between the bearing portions. At least one of the bearing portions is mounted on the device so as to be readjustable so that a position of the bearing portion is modifiable to align the first furniture part in the assembled state in a horizontal direction in relation to the motion rail.