When a reader of a Radio Frequency Identification (RF-ID) system sends out a command signal to the RF-ID tag and receives a response signal from the RF-ID tag, an End of Signal (EOS) of the received response signal is detected such that the next command signal can be transmitted within a time. Accordingly, response signal can be detected even under the environment of poor signal to noise ratio performance, and the EOS of the response signal from the RF-ID tag can be recognized with ease and speed.