To provide a remote optical detector that is superior in durability, dispenses with the need for power supply, durable against noise, such as lightning surge.

The light from the light source 25 provided in the monitoring device 20 apart from the monitoring object is transmitted to a sensor part 30, provided in the monitoring area through optical fibers 50 and 32, and is then emitted from a light-emitting part 33 to the sensor part 30 to irradiate the rotating body 36. The rotating body is provided with translucent slits 40 at the periphery, when the rotating body is rotated, a light-receiving part 34 receives the light transmitted through slits 40. The received transmitted light is sent by the optical fibers 35 and 50 to the detection part 26 of the monitoring device 20. The detection part calculates the amount of the rotation of the rotating body 36, by counting the number of slits through the receipt the transmission light. For example, at the detection of a landslide, with one end of a wire 39 is wound on the rotational shaft 37, with the other end of the wire 39 fixed on the observation object region, if the landslide occurred, the rotating body 36 is rotated i.e. the amount of the landslide is converted into rotational motion.