PURPOSE:To prevent a fire sensor from being mischievously detached by providing a lock mechanism consisting of an L-shaped spring between a fire sensor base and the fire sensor which are fitted to each other via a bayonet. CONSTITUTION:A fire alarm 4 is attached to a fire sensor base 1 which is already attached to the ceiling, etc., of a room. That is, the alarm 4 is put on the base 1 and at the same time the alarm 4 is slightly turned toward an arrow A4. Then the connection terminal of the alarm 4 is connected to the connection terminal of the board 1 via a bayonet. Under such conditions, a projection part 5a of an engagement part 6 of an L-shaped spring 3 provided on the base 1 is deeply inserted to a hollow 5b of an engaged part 5 of the alarm 4. Thus the bayonet engagement state is locked. At the same time, the alarm 4 is prevented by a stopper 16 from moving more toward the arrow A4. Thus the part 5a of the spring 3 is never damaged.
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