A distance monitoring device is provided. The device is suitable for a chemical mechanical polishing (CMP) apparatus. A polishing head of the CMP apparatus includes a frame and a membrane. The membrane is mounted on the frame, and a plurality of air bags is formed by the membrane and the frame in the polishing head. The distance monitoring device includes a plurality of distance detectors disposed on the frame corresponding to the air bags respectively to set a location of each of the distance detectors on the frame as a reference point, wherein each of the distance detectors is configured to measure a distance between each of the reference points and the membrane.