To detect an abnormal heat build-up in a transmission line and an electrical equipment accurately and to display it.

A wax-type thermostat is buried into a body made of a metal that consists of a screw part 3 where a thread is cut the outer periphery and a cylindrical part 4 that is formed into one piece at the head part and a piston rod 10 is positioned in the cylindrical part so that it can be expanded and contracted freely, a cylindrical display body 11 where the upper edge is blocked by a top 12 covers the piston rod so that the top 12 is located above, and a spring 15 is included between the reverse side of the top 12 and the inner bottom of the cylindrical part 4, thus energizing the display body 11 upward. Then, a gear-lock plate 16 is mounted to the surface side of the top 12, the edge part is tentatively locked by a washer 7 being mounted to the upper edge inner periphery of the cylindrical part 4, the display body 11 is accommodated in the cylindrical part against the elastic force of the spring 15 when the piston rod 10 shrinks, the tentatively locked state between the gear-lock plate and the washer is canceled due to the press force when the piston rod 10 is extended, and the display body 11 is allowed to protrude upward from the cylindrical part 4 due to the resilience of the spring 15.