A fire detector consisting of a body and a base. An inner wall part is provided in a circumferential wall of the body in such a manner that the inner wall part can be cut away therefrom. Further, an engaging member abutting on the inner wall part is provided in the base. If it is unnecessary to lock the body to the base, the inner wall part is not cut away therefrom. In contrast, if it is necessary to lock the body to the base, the inner wall part is cut away therefrom. Thereby, the engaging member 12 can be fitted into a recess portion formed on the circumferential wall by cutting away the inner wall part therefrom. If the body is pushed against the base, a leading end of the engaging member is pushed up by the top surface of the circumferential wall. When the body being in such a state is turned in the circumferential or clockwise direction thereof to a fixing position at which the body is fixed, the leading end is fitted into the recess portion and moves downwardly. Even if the body being in this state is turned to an initial position thereof and is then detached therefrom, the leading end abuts the recess portion in the circumferential direction thereof and thus comes to be unable to be turned. Namely, the body is locked to the base thereof. Thereby, it can be selected according to the specifications of the fire detector as to whether or not the body is to be locked to the base. Consequently, the fire detector of the invention can be formed at a low cost regardless of whether or not the body is to be locked to the base of the fire detector.