PURPOSE: To detect the development of local cracks with a sufficiently high sensitivity by detecting cracks by monitoring a variation of a transmission characteristic of an optical fiber joined and stretched around the surface of the concrete construction.

CONSTITUTION: A winding of an optical fiber 12 is stretched around the outer surface of a concrete LNG tank 10 and the optical fiber 12 is adhered to the surface of the tank with adhesives. A terminal of the optical fiber 12 is connected to a light source 14, from which a light of constant strength is projected to the other terminal of the optical fiber 12 is corrected to a photo-receiver 16, through which the light projected from the optical fiber 12 is O/E converted. The output of the photo-receiver 16 is input to a level discrimination circuit 18, the level of luminous energy projected into the photo-receiver 16 is constantly monitored whether reaching a standard level or not, the optical fiber 12 is made of silica glass and consists of a core 12a, a clad 12b and a coating 12C.