PURPOSE: To monitor the abnormal temp. of a bath duct without requiring a power supply or wiring.

CONSTITUTION: A bimetal 1, the slide plate 2 integrated therewith and the alarm strip 4 fixed to a sensor box at one end thereof and having a dead weight 5 attached to the other end thereof are provided in the sensor box. The weight support plate 3 of which the end part is supported by the slide plate 2 is provided in the sensor box by hinges and the weight 5 is placed thereon. When the bimetal 1 warps by a temp. rise, the slide plate 2 is moved and the weight support plate 3 revolves downwardly corresponding to the movement of the slide plate 2 to allow the weight 5 to fall and the arm strip 4 is drawn out and, from the suspended state of the alarm strip 4, abnormal generation of heat is detected.