To provide a heat sensitive wire capable of quick operation, mounted on electronic/electric devices so as to contact with the same, operated by the overheat thereof.

A heat sensitive insulation layer 2 is formed on an internal conductor 1, an external conductor 3 is formed on the heat sensitive insulation layer 2, and a protection cover layer 4 is formed on the external conductor 3. The relation of a diameter d of elemental wire of external conductor 3 to the thickness h of the protection cover layer 4 is d≤30h, and product of the thickness h and the cross section S of the heat sensitive wire 2 is S×h≤14.0×10-3. Time constant of temperature increase of the heat sensitive insulation layer against the heating from outside thereof can be made sufficiently large, and the protected electronic/electric device can be protected thermally safely by stopping power supply after the start of overheat thereof.