A detector for detecting a relative position between a rope (2) and a guide member (13) in a transfer apparatus including the rope (2) travelling by itself or fixed to a support; the guide member (13) such as a pulley engaging with the rope, and fixed to the support or travelling by itself; a transfer member travelling with the rope (2) or with the guide member (13); and a displacement detector (1) for detecting a relative position between the rope and the guide member, fitted in the proximity of the guide member (13) and keeping a gap with the guide member always constant; wherein the displacement detector (1) includes medium generation means for generating a position detection medium emitted from the detector towards the rope (2) and detection means for detecting the displacement of this medium. The displacement detector (1) includes speed control means (25) for driving the transfer member, and the speed control means (25) controls the speed of the transfer member in accordance with displacement signal from detector (1).