The invention to the optical transmission unit without using a control line providing a separated sensor capable of synchronizing and the light receiving unit. That is, there is provided a separated sensor including a light receiving unit that receives the detection light transmitted by the light transmitting unit and the light transmitting unit that transmits detection light to a monitoring area, and the light transmitting unit and the light receiving unit is to separate from each other It is arranged. The unit of any one of the light transmitting unit and the light receiving unit, there is provided a synchronization-light transmitting unit that transmits synchronization light of a light transmitting unit to the light receiving unit and the synchronization over the air. The light transmitting unit and the remaining one unit of the light receiving unit, predetermined to a synchronization based on the synchronization light received by the synchronization-light receiving unit and the synchronization-light receiving unit that receives the synchronization light transmitted from the synchronization-light transmitting unit run a synchronization-establishment processing unit that performs a process is provided. Separated sensor, the light transmitting unit, the light receiving unit, the synchronization-light transmitting unit, the synchronization-light receiving unit