An object is to build a protective structure which can protect a ferroelectric material from a surrounding environment when the ferroelectric material is used as a heat detecting element of a heat detector, thereby enhancing the reliability of the heat detector.A heat detector includes a heat detecting unit 11 that senses heat in a monitoring area, and a casing 81 that holds the heat detecting unit 11, and the heat detecting unit is held by the casing 11 via a laminating portion 60 which serves to protect the heat detecting unit 11.