To provide a method for accurately detecting falling stones on a slope by using a sensor system for measuring strain for an optical fiber advantageous to a system building surface.

Falling stones 31 are guided to a storage site by a prevention net 1 for the falling stones lined on the slope, and the bulge of the net 1 is detected by the optical fiber 3 positioned along the net 1. The optical fiber 3 is stretched by the bulge of the net 1, and the bulging of the net 1, the generation of the falling stones, is determined when strain by elongation in this case is measured. The optical fiber 3 is stretched by the weight of the falling stones, and strain can also be generated.