A communication device (101) transmits measurement result information (110) representing the number of steps taken by a farm animal (A) to a repeater (102) located in a communication area (103). When the repeater (102) receives the measurement result information (110) from the communication device (101), the repeater transmits the measurement result information (110) to an estrus notification device (104). For each group, the estrus notification device (104) stores the communication devices (101) fitted on each farm animal that forms the group in association with the GPS communication device (107) fitted on one of the farm animals that form the group. When it is determined that the measurement result information (110) received from the repeater (102) meets a specified condition, the estrus notification device (104) searches for the GPS communication device (107) associated with the communication device (101). The estrus notification device (104) transmits start instruction information (130) to the GPS communication device (107). When the GPS communication device (107) receives the start instruction information (130), the GPS communication device turns the power supply ON.
