To provide an econimical device having a simple structure even for a variety of objects to be monitored.
A plurality of chained sensors 20 are connected on an optical transmission route 41 formed of optical fiber, and photodetectors 42 are connected to both ends of the route 41, further the respective output sides of the detectors 42 are connected to a signal processing means 45. The temporary decrease or increase in light intensity signal generating when an object 2 to be monitored passes through between the sensor 20 and an LED 46 is transmitted toward both ends of chained sensors, and it arrives at respective detectors 42 by a delay of time depending on the length of route. The means 45 identifies the sensor 20 where any change occurs due to level changing quantity and timing difference of level changing, and as a result, the passage of the object 2 at the position of the sensor 20 is detected.
YODA MASAKI
TANAKA KOTARO
MATSUBAYASHI YOSHIKAZU
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