To prevent the occurrence of fault caused by the discharge of low temperature melting material to the outside by providing a heat resistant air filter on a route for melting and discharging the low temperature melting material to the outside together with high pressure air inside a thermosensitive device.

One end of a heat conductive cylinder A is linked to a high pressure air conduit E, and high pressure air is introduced from the high pressure air conduit E. A hollow cylinder C such as a glass pipe is linked through a hollow heat insulating cylinder B to the other end. The inside of the heat conductive cylinder A is sealed with a low temperature melting material D. The inside of the cylinder C is partitioned with a heat resistant air filter F composed of heat resistant fibers such as glass fibers. When an object to monitor overheat is heated over, the low temperature melting material D in the heat conductive cylinder A is melted, sealing in the heat conductive cylinder A is canceled and high pressure air flows to the heat resistant cylinder B and cylinder C. Since the low temperature melting material D is caught by the heat resistant air filter F, it can be prevented from being discharged to the outside.