An edge detecting apparatus for using in an automatic device and configured for detecting edges of a surface on which the automatic device moves is provided. The edge detecting apparatus includes an emitter, a guiding member, and a receiver. The emitter is configured for emitting projecting signal. The guiding member is around the automatic device. The guiding member and the surface define a signal channel, and the signal channel is configured for guiding the projecting signal to travel around the automatic device, if no edge of the surface exists under the signal channel. The receiver is configured for receiving the projecting signal.