Abstract: In a system for identifying a plurality of cattle bred on a livestock farm and detecting an estrus state thereof, an apparatus for detecting and identifying any one of cows in the estrus state, comprising a switch mounted on a caudal part of a corresponding one of the cows in such a manner that it can be operated when a different one of the cows mounts the corresponding cow, an encoder for generating an inherent identification code with respect to the corresponding cow in response to the operation of the switch, a radio transmitter/receiver circuit for transmitting the inherent identification code from the encoder externally by radio and receiving an external identification code by radio, a decoder for decoding the external identification code received by the radio transmitter/receiver circuit to detect an identification code value therefrom, a comparator for comparing the identification code value from the decoder with a prestored identification code value of the corresponding cow to determine whether the tw