To regularly monitor underground state of an anchor construction part to judge a change in freezing status or a position of underground water by utilizing an optic fiber as sensor to measure distortion and temperature along the optic fiber.

An anchorage device 4 is held by a top plate 3 with a cap 2 put on in an anchor structure, and a PC steel material 5 is screwed into the anchorage device 4. A half on the head side of the steel material 5 is a free length part covered by a sheath 6, and its tip is an anchorage length part filled with grout. A fixing device 8 is attached to a predetermined position of the steel material 5, and a sensor fiber 1 is wound around its outer periphery. That is, the sensor fiber 1 is inserted from the head of the anchor structure along the steel material 5 and folded at a semi-cylindrical member 20 adjacent to the fixing device 8 on the free length part, and an end of the sensor fiber 1 is pulled out from the head of an anchor. Thus, when there is a sliding surface in the surrounding ground, both distortion and temperature can be monitored to sense their existence.