A reception unit 2 of a detection unit 3 includes a transmitter 30 that transmits a reception (detection) level of an infrared beam to a transmission unit 1, and a demand signal output unit 26 that transmits to the transmission unit 1 a demand signal M, demanding to control the intensity of the infrared beam to be transmitted so that the reception level matches a predetermined value. The transmission unit 1 includes an infrared ray (detection beam) intensity control unit 15 that controls, upon receipt of the demand signal M, the intensity of the infrared beam to be transmitted so that the reception level matches the predetermined value. A power source unit 31 is a solar battery unit including a solar panel and a charging medium that stores power from the solar panel.