An entry detecting system includes a terminal device; a point-to-point communication device; an entry detecting device; and a guard device. The point-to-point communication device includes a transmitting-receiving antenna having directivity in a specific direction. The point-to-point communication device communicates by radio with the terminal device via the transmitting receiving antenna so as to detect a response time from the terminal device and report detected response time to the entry detecting device. The entry detecting device detects a distance from the point-to-point communication device to the terminal device based on the response time reported by the point-to-point communication device, generates entry detecting information based on the detected distance, and transmits the entry detecting information to the point-to-point communication device and the guard device. The point-to-point communication device transmits the received entry detecting information to the terminal device.