To provide an animal approach detection system for accurately detecting whether an animal comes beyond a border for stopping the invasion of the animal or not.

The animal approach detection system is characterized by having an antenna 2 for receiving electric waves transmitted from a transmitter 1 attached to an animal, a direction detection means 6 for detecting the direction of the transmitter 1 to the antenna 2, and an approach state-determining portion 35 for memorizing standard receiving intensity information obtained by relating a direction to a standard receiving intensity of an electric wave received by the antenna 2 from a movable transmitter ST, when two standard transmitting points po1, po2 are set in different directions, while the antenna 2 is used as a base point po0, and when the movable transmitter ST having the same electric wave characteristics as that of the transmitter 1 is moved on a border BD1 established to positions for binding the standard transmitting points po1, po2, obtaining a standard receiving intensity corresponding to a direction detected with the direction detection means 6 from the standard receiving intensity information, and then comparing the obtained standard receiving intensity with the receiving intensity of an electric wave received from the transmitter 1 to determine whether the animal to which the transmitter 1 is attached has approached on the standard point po0 side beyond the border BD1.