PURPOSE: To highly accurately detect abnormal states such as flood or fire on the bottom part of or ceiling of a tank by means of simple constitution by fitting a shape changing jig for converting a change in temperature or humidity around an optical fiber into physical bending force to the optical fiber in response to the change to the optical fiber.

CONSTITUTION: The shape changing jig 1 is fitted to the optical fiber. A light source for transmitting the prescribed quantity of light to the optical fiber is connected to one end of the optical fiber 2 and a light quantity measuring instrument for measuring the quantity of light transmitted to the optical fiber is connected to the other end. When the temperature is raised up to 50W60°C, the jig 1 is deformed to a curved shape. When the deformation of the jig 1 is generated, the optical fiber 2 is forcedly bent and the quantity of light flowing into the optical fiber 2 is lost. Thus, the abnormal state such as flood or fire can be remotely sensed. A chemical reaction layer 6 generating a heat generating or heat absorbing reaction at the time of coming into contact with water may be arranged on the shape changing jig 1.