PURPOSE: To obtain a detecting element, whose training is easy and which can be used safely even under the atmosphere of a dangerous material by using an optical fiber, which uses a member made of plastic that is deformed or broken with heat, as a heat sensitive part, and using connectors attached to both ends of the optical fiber.

CONSTITUTION: An optical fiber 1 made of plastic is a part, which functions as a heat sensitive element. Connectors 2 for the optical fiber are used for connection to other optical fibers 3 made of quartz. One end is connected to a light generating device, and the other end is connected to a light receiving device. When the optical fiber 1 made of plastic is deformed or melted by the heat caused by fire light emitted from the light generating device is attenuated to a level lower than the normal case, and the attenuated light reaches the light receiving device or the light does not reach at all. Thus, the occurrence of the fire can be detected.