PURPOSE: To obtain a large mechanical displacement to the variation of the surrounding temperature and to make it possible to operate switches with no error operation by using a shape memory alloy to operate the switches.

CONSTITUTION: One end of a shape memory alloy 2 is installed to the ceiling surface between two switches 3, and the other end is installed between projections of an operation plate 1. Although the shape memory alloy 2 holds the operation plate 1 presenting the form of an extended spring at the room temperature, it is contracted and it remembers the form to operate the switches 3 through the operation plate 1 when the surrounding temperature becomes a high temperature set beforehand. As a result, when a fire breaks out, for example, the shape memory alloy is contracted by the heat, the operation plate 1 is lifted upward, the switches 3 are pushed by the operation plate 1 to turn on, and a fire fighting unit control board gets the delivered information that the surrounding temperature comes to be a preset temperature through signal lines 4. Consequently, a large mechanical displacement is obtained to the variation of the surrounding temperature, and the switches can be operated without any error operation.